Do you have a crib for baby that you put him/her into after the first several weeks? If so, I'd get the Fisher Price Rock and Soothe bassinet. It is super light weight, folds up, and keeps baby in a cuddled 45 degree position. It was the best baby gear I bought. Perfect for an infant up to 4 months (depending on how quickly baby learns to start rolling over) It is $60 new. I bought mine 2 years ago for $50 and sold it for $35. It was worth the $15 I ended up paying for it.
I'd put baby in the Fisher Price bassinet any day over using a bassinet in a pack and play. Less space and more comfortable for baby (especially if he/she is a spitter or has acid reflux).
That buys you a couple of months before you need to "make" the 2 year old sleep in her own bed. I know easier said than done, but it can be done. :) If you can, try to get her to stay there now, before baby comes and you are not sleep deprived. Even if she regresses (and she will), you have laid down the foundation of what you expect.