M.O.
M.,
What about putting the baby in a pack n play somewhere else - or will she just climb out?
I agree with the gate idea too, however it doesn't stop the night waking/getting out of bed. I would come up with some reward that she REALLY loves to see if that will motivate her to "roll over and go back to sleep". She's old enough to understand that it makes Mommy, Daddy and sister "sad" when she wakes them at night. Give her some "big girl" reward, that doesn't involve baby sister...lunch out with Mom or Dad? Running errands without the baby? A special video loaned from the library? (i.e. t.v. time) We've been through this and consistency is the key.
I love how sometimes the Dads have ideas, but don't have to go through getting up all the time to help out. You need to make sure you "tag team" with your husband. Let him take the little one, and you enforce bedtime rules with your older daughter.
Consistency is the key. Stick to your guns. YOU need to show her that her screaming and carrying on is NOT acceptable, nor will you give in.
Good luck!