It happens. Especially since she's probably still feeling frustrated about having to share mom and dad, biting is definitely a satisfying way to show frustration- take that! LOL! She'll get used to it, soon enough she won't remember a time when she didn't have a brother :)
I definitely don't want to be critical of the good lady since I'm sure she is a great lady, but I would be wondering how the fuck i was the bad guy if I was the one who got bit too ;)