N.S.
This is not uncommon. Heck, I even get potty issues with my 8-year old when she's having trouble at mommy's with little sister!
We've always told her that she gets special privileges because she is older, but only if she ACTS like an older sister. We make a big deal out of these (they include staying up late, having special time with daddy, etc). If she regresses we take those privileges away, making sure she knows. She has to go to bed really early. She can't have "big girl" dessert. And we ignore some behavior (like baby talk) and let her know when she's ready to act her age we will pay attention to her. You could try completely ignoring the hitting and biting. Don't look at her, just move away. You could mention once (I'm not going to ___ for you until you start behaving) and then ignore. If hitting and biting doesn't get the intended result (attention) she'll stop. She will probably turn up the drama at that point, but then you know it's working. Simply continue to ignore and she'll sort of blow up, then stop. She's only continuing to hit and bite because it gets a reaction and she's decided it's a solution for her. So...make it so it's not a solution.
Also, letting her know her role helps too. Sometimes kids don't understand their new role, all they know is what they used to do (which is be a baby) and they see their sibling acting like a baby and getting attention. Giving her jobs and helping her to know what a Big Sister does seems to help. I don't think it's just the attention they need, but also they need to feel needed.
Perhaps putting her in "charge" of teaching your youngest to potty train will help? Suddenly she will feel older and needed, and I would bet the accidents would stop! She will want to be the example.
Good luck!