M.F.
Making sure she gets "mommy time" is important. But it's still going to happen. You might think about having some sort of standard consequence and a "catch phrase" for being too rough with the baby. Then separate them. When she does it, maybe say things like, "Oh no, looks like you're being too rough with the baby. You probably need a little break to remember how to be gentle. Please go to your room. You have 3 minutes to think about ways you can be sweet to your sister. You can tell me everything you think of when the timer beeps."
I started sending my oldest to her room every time she was rough or mean to her baby brother and it didn't take long for her to have "nice hands."