We had the same issue with my son. He was 3 when we had our daughter.
Keep in mind your daughter has had a BIG change in her life with an 8 week old in the house. When all else is beyond the control of a child there are 3 things he/she can control: eating, sleeping and eliminating. So there is a tendency to see issues with at least one of those when a major change (like a baby, or a move or starting school occur).
We didn't make a big deal about pooping issues. Did a lot of postive reinforcement with sticker charts. But when it came down to it--he started pooping in the potty when he was feeling more in control of other areas of his life. Actually allowed him to put a "pull-up" on to poop in (got tired of cleaning poopy underwear--yuck).
D.