Its hard isn't it, we moved last year (so DD#1 was 3.5) and she STILL mentions the old house.
I find that just letting her talk about it, and looking at pictures of the old place, and remembering good things about the old place helped - because then I could ask her about some of the fun times she has since had in the NEW place, and that we are making lots of happy memories here.
I also talked about how it was good that we left the old place, because now someone else has somewhere to live.
Just keep reassuring her, and let her talk about it.
WRT the new bub - keep talking about that too - kids are resilient and can cope better than we think most of the time

All the best!