M.Y.
I'm in the same boat as you are right now. I've got a 2.5 year old boy and a 6-month old daughter. My little boy climbed into her bassinet endlessly. All I could say is that with CONSTANT reinforcement of giving him time outs when he didn't listen, he now does not climb into her bassinet. I gave him 3 warnings and then a timeout after he continued the behavior. This worked for me. During the night, I would try reinforcing verbally to your toddler and let her know that she should not go into baby's crib. I know that when we continuously drill something into our toddler, he seems to get it. Good luck!