Here is some homebrew that I would like to use for the two sisters.
Carnal Mage(Modified Beguiler) - Updated for Pathfinder
Spells: To cast a Carnal Mage spell, you must have an Charisma score of 10 + the spellís level (Cha 10 for 0-level spells, Cha 11 for 1st-level spells, and so forth). The Difficulty Class for a saving throw against a Carnal Mageís spell is 10 + the spellís level + the Carnal Mageís Cha modifier. Like other spellcasters, a Carnal Mage can cast only a certain number of spells of each spell level per day. The base daily spell allotment is given on Table 1Ė1(Beguiler, PHB II). In addition, you receive bonus spells for a high Charisma score
A Carnal Mage needs a full night's rest and regains her spells through having sex for an hour(this can occur before or after resting). Masturbation does not count as sex for this purpose. This would use the mechanics for Perform: Sexual Techniques in the Book of Erotic Fantasy.
Class Skills (6 + Int modifier per level): Acrobatics, Bluff, Climb, Diplomacy, Disguise, Escape Artist, Intimidate, Knowledge (Arcana), Knowledge (Local), Linguistics, Perform (Sexual Techniques), Perception, Profession, Sense Motive, Sleight of Hand, Spellcraft, Stealth, Swim, Use Magic Device.
BAB/Saves/Spells Per Day(Aside from 0 level): Same as Beguiler
Cantrips: 0 Level Spells are cast like any other spell, but
they are not expended when cast and may be used again.
Eschew Materials: The Carnal Mage gains Eschew Materials as a bonus feat at first level.
Orgasmic Renewal (Ex) (Replaces Trapfinding): The Carnal Mage can convert the sexual energy produced by an orgasm into magical energy to renew spell slots. She can only do this once per day at first level, gaining an additional use every-other level(3rd level, 5th level and so on for a total of 10 times per day at 19th level).
Masturbatory Renewal At first level, the Carnal Mage can renew a 1st Level Spell Slot by achieving an orgasm through self stimulation.
Sensual Caresses At fifth level, the Carnal Mage can renew up to a 2nd Level Spell Slot by achieving an orgasm through the sensual massages of a partner's hands
Oral Bliss At ninth level, the Carnal Mage can renew up to a 3rd Level Spell Slot by achieving an orgasm through her partner's blissful application of lips and tongue.
Erotic Delight At thirteenth level, the Carnal mage can renew up to a 4th Level Spell Slot by achieving an orgasm through the delightful joining of her partner's genitals with her own.
Complete Ecstasy At Seventeenth level, the Carnal Mage can renew up to a 5th Level Spell Slot by achieving an orgasm through her partner combining Sensuality, Bliss and Delight to induce pure ecstasy.
Distracting Tease (Ex) (Replaces Armor Proficiency and Armored Mage): Often a good distraction can provide an effective defense. A peeking areola, partially exposed southern cheeks or the subtle press of puffy petals against thin snug fabric can be all it takes to cause an opportune loss of focus. At first level the Carnal Mage gains a bonus to her Armor Class equal to her Charisma Modifier. This ability functions only when the Carnal Mage is wearing revealing clothing.
Cloaked Casting (Ex): Starting at 2nd level, a Carnal Mageís spells become more effective when cast against an unwary foe. You gain a +1 bonus to the spellís save DC when you cast a spell that targets any foe who would be denied a Dexterity bonus to AC (whether the target actually has a Dexterity bonus or not).
At 8th level, you gain a +2 bonus on rolls made to overcome the spell resistance of any affected target.
At 14th level, the bonus to your spellís save DC increases to +2.
At 20th level, you become able to automatically overcome the spell resistance of any affected target.
Surprise Casting (Ex): Starting at 2nd level, when you successfully use the Bluff skill to feint in combat, your target is denied its Dexterity bonus (if it has one) to AC for the next melee attack you make against it or the next spell you cast. You must remain in melee with the target, and the attack must be made or the spell cast on or before your next turn. The target is not considered fl at-footed and therefore can make attacks of opportunity against you if you do not cast defensively. Surprise casting can also be used when engaged in a sexual act(including masturbation).
At 6th level, you gain the ability to feint in combat as a move action instead of a standard action. If you have the Improved Feint feat, you can now feint in combat as a swift action.
Advanced Learning (Ex): At 3rd level, you can add a new spell to your list, representing the result of personal study and experimentation. The spell must be a sorcerer/wizard spell of the enchantment or illusion school and of a level no higher than that of the highest-level spell you already know. Once a new spell is selected, it is forever added to your spell list and can be cast just like any other spell on your list. You gain another new spell at 7th, 11th, 15th, and 19th level
Silent Spell: At 5th level, you gain Silent Spell as a
Still Spell: At 10th level, you gain Still Spell as a bonus
Carnal Mage Spell List
0 Level Spells: Depliatory, Daze, Detect Magic, Ghost Sound, Grope, Message, Prestidigitation.
1st Level Spells: Block the Seed, Charm Person, Comprehend Languages, Disguise Self, Expeditious Retreat, Hypnotism, Mage Armor, Obscuring Mist, Share Sensation, Silent Image, Sleep, Undetectable Alignment, Visions of Exquisite Pleasure, Whelm.
2nd Level Spells: Daze Monster, Detect Thoughts, Disrobe, Fog Cloud, Glitterdust, Hypnotic Pattern, Invisibility, Magic Probe, Minor Image, Mirror Image, Misdirection, See Invisibility, Silence, Spider Climb, Stay the Hand, Touch of Idiocy, Touch Me Not, Unseen Lover, Whelming Burst.
3rd Level Spells: Arcane Sight, Clairaudience/Clairvoyance, Cursed Orgasm, Deep Slumber, Dispel Magic, Glibness, Halt, Haste, Hesitate, Hold Person, Inevitable Defeat, Invisibility Sphere, Major Image, Nondetection, Phantasmal Lover, Privacy, Reverse Gender, Slow, Suggestion, Zone of Silence.
4th Level Spells: When Needed
5th Level Spells: When Needed
6th Level Spells: When Needed
Spell List Tradings
0 Level: Dancing Lights for Depliatory(BoEF pg 102), Open/Close for Grope(BoEF pg 106), Read Magic for Prestidigitation
1st Level: Color Spray for Visions of Exquisite Pleasure(BoEF pg 117), Detect Secret Doors for Block the Seed (BoEF pg 101), Rouse for Share Sensation (BoEF pg 115)
2nd Level: Blinding Color Surge for Magic Probe(BoEF pg 111), Blur for Disrobe(BoEF pg 103), Knock for Touch Me Not(BoEF pg 116-117), Vertigo for Unseen Lover(BoEF pg 117)
3rd Level: Displacement for Phantasmal Lover(BoEF pg 113), Crown of Veils for Cursed Orgasm(BoEF pg 102), Legion of Sentinels for Privacy(BoEF pg 114), Vertigo Field for Reverse Gender(BoEF pg 115)
4th Level: Coming soon, when needed
5th Level: Coming soon, when needed
6th Level: Coming soon, when needed
Nymph Blooded Template
Captivating Beauty: When looking at the individual in their full glory(nude or mostly), Will Save or Dazed for one round. Can't suppress or renew.
Nature's Grace: Wild Empathy(As Druid ability) with +2 Racial Bonus(Nymph is +6 racial bonus, roughly one-third nymph, one third the bonus, heh). Knowledge (Nature) and Survival always considered class skills.
Nymphomania(Flaw): Certain parts of the body are more sensitive than normal, usually breasts(for women) and genitalia that are affected. This causes the individual to become aroused rather easily depending on the body part and level of increased sensitivity. Along with increased sensitivity are intense periodic cravings of certain forms of stimulation, usually a form of stimulation the individual hasn't experienced in awhile.
For men this problem occurs about once a month, for women it tends to take affect right after their normal monthly cycle. The effects between both genders are the same however, depending on the type of stimulation, certain parts of the body will begin to tingle and itch differently. It will be very minor at first, eventually growing into a full on aching need after several days. This is the first clue to what their body is craving, the second is that soon after their period starts(even the first night), they're treated to increasingly vivid and intense erotic dreams. Once their nymphomaniac period begins, each masturbatory orgasm becomes less and less satisfying until they find that they can no longer bring themselves to a climax with self stimulation. After a couple of days this problem starts affecting other forums of stimulation until the craving is fully satisfied.
The intensity of the craving and resulting effects gets worse and worse with each passing day until the craving is satisfied. While the first day may not be all that different aside from being constantly horny, the fifth day has their bodies in a never ending state of full arousal.
So that's pretty much the fluff. as for mechanics I'm not sure how exactly to handle it. Thinking a table for the type of stimulation craved(which would go along with the character's kinks and limits, for example; if the character did not like anal, anal stimulation wouldn't be a possibility). As well as a cumulative penalty/bonus for each day that passes without having the craving satisfied. Penalties would likely affect most skills(especially those that require concentration) due to the distraction of intense arousal and desire. This would also affect spell casting, and maybe even attack rolls. However, bonuses would likely be given to diplomacy(only when seducing) and to Perform: Sexual Techniques. Thinking that the penalty would be increased by one for each day that passes. First day(start of the period) would be -1/+1, the fifth day being -5/+5, and so on.