Honestly, this is very normal. My 3yr old daughter is constantly playing with imaginary friends of all sorts. OK, we probablly need to stop her animating her green beans at dinnertime, but mostly it's harmless.
On a more serious note, she is developing a very important skill by doing imagiantive play. It's helping her develop executive function, which is the ability to self regulate. She is subjegating herself to a set of rules she makes for herself. This will develop into time management and self discipline in the long run.
Its also a way for my daughter to process her life right now. She will cast herself and her imaginary friends in plots of books or videos that she has been exposed to and goes through them repeatedly. It's actually a lot of fun to listen to and watch.
Oh, and as a final reassurance, this is normal, not an outgrowth of having a new sibling. My daughter started this befor the arrival of our son who is now 8 months old. I admit that at first I was concerned that I was not meeting both of my children's emotional needs. I'm not meeting all of them, by any means, but I'm meeting enough for them both to grow and be healthy. Thus, I know I can get rid of that mommy guilt. Tell your daughter she can relax and sit back and watch her daughters mind grow through her imaginary play.
S.