Boy, this is a toughie! I think I'd be inclined to talk to your ped about it.
since little sib is 2 months old I think it would be OK to transition to a BGB. I realize that it is still a bit crazy, but it's not a brand new idea any more. And now that sib is here you can make a huge party about it and say that it's her reward for being a big sister! Go shopping together for new bedding, buy a new 'sentry' animal to watch over her at night, and just really celebrate the change. There's also some great books out there about sleeping in big kid beds that can help prepare her for the transition.
I have a feeling now it's more of a habit than anything. It probably started due to stress and now it's just kind of a physical habit. Breaking it may take changing beds.