They had always been raised to be comfortable with nudity, their parents always naked around them. Of course, over the years, this led to several embarrassing incidents, like walking in while their parents were having sex or were in other such situations. Both sisters (either Twins or a year apart) had certainly seen their dads cock plenty of times over the years, soft and aroused. Things had changed once their mother had moved on, leaving them with just their father. He seemed so sad these days because of it. They had seen how happy he had been with their mother so they decided that now that she was gone they had to fill her place. And who could resist to beautiful girls that loved him and wanted to make him happy? The way he got aroused when he saw them naked certainly hadn't escaped their notice either.
The girls (either twins, or a year apart) had been groomed their entire life. Comfortable nude, they had seen their parents having sex plenty of times right in front of them. When they had first started dating boys, their mother had showed them the proper way to give a boy a blowjob - using their father as an example. Both girls agreed that their father was definitely the most handsome man they had ever seen and he didn't cum so quickly like all the other boys did. With their mother away for the next month on a major expedition to Africa (she was a scientist) their father would be all alone. With her gone they decide that they can trick their father into helping them practice their 'skills' on him and he certainly isn't about to say no.
Either some one to play both girls, or two different people to pick up each role. If you are interested in this, let me know which idea you'd be interested in. Might as well post here. Once I get 2 people for whichever version (and I want both versions) we can get started.