No Ghost/Movie ending
I don't think the entity appearing to Mary and her mom was Jennifer's ghost. I believe it was a demon impersonating the dead girl. The movie seems to hint that Jennifer was not very nice to her sister and maybe even her brother sometimes, so that is how the demon represented Jennifer.
I think both parents never came to grips with how devious "Jennifer" appeared to the mom, the mother was afraid, finally coming to grips with how devious she was, but of course she believed it was really her dead daughter appearing.
I didn't like the movie ending the way it did. Should have ended with the mom going to Mary, apologizing for all she and her husband has let Jennifer do to her and to help Mary see what the mom now knows: They must fight this evil and defeat it.