Sister wants to know how I'll explain it to my daughter ?

I'm 25 weeks pregnant and my sister is lesbian and married to her wife. Today she asked me how I'll explain it to my daughter because she wants to know and hear to approve. I'm just super confused by this I don't think I'll explain anything unless my daughter ask, which I know she probably will eventually. I also don't think she can "approve" what I'll tell my daughter.