I have a totally different take on Isabelle, whether she's in utero, toddler or adult. I don't think her abilities she demonstrated while Lily was alive were really Isabelle's doing at all. I think Lily herself caused most of the supernatural stuff to happen and blamed it on Isabelle. Did you ever notice that this stuff happened only in the presence of Lily? When Heidi got sick, it was after spending the afternoon with Lily. Even fortelling Jordan's fate, Lily was only a few feet away from him.

I think Isabelle is acting differently because Lily is dead. Sort of like when the suspect dies, the crimes stop. I'm sure adult Isabelle has her own powers, but I don't think they are the same as baby Isabelle's.