She could just replace
Amelia. Afterall, while she was important to Viktor's powerbase, its never really explained why. Marcus & Viktor we can leave in place to form background history for the game and since I'm only allowing 1 Elder, Amerlia is pretty much up for AU-ing. It might make your life a bit easier.
My character is replacing Lucian after all.
As for why
Marcus might have made her a Vampire, I'd suggest something to do with resources. He turned Viktor for an army, why not turn a woman who's released deceased husband was one of Hungary's finest merchants or owned a silver mine?
If she became a Vampire before
Viktor why wasn't she an Elder in the first place? She's also about 92 years too old, if Marcus is from mid to late 5th century and Viktor was turned in about 500 A.D. or slightly before, and Amelia after that. I also don't see how an Amazon comes into things. Seems like she's from a place pretty distant to wind up in Hungary.
I wouldn't worry about the character being OP. Lucian was able to hold his own (and defeated) Viktor and Amelia was killed by Raze. So while its expected she'll be physically superior to her younger Vampires and younger Lycans, she'll still be challenged. Lycans from Lucian/Adam's time will still pose a very real threat.