It sounds like you are a very loving aunt and she has loving parents too. I would be wary of suggesting there is something wrong and instead be there to support your niece and her parents in whatever route their journey is going on. The chances are soon she will go to kinder or preschool and at that point, if there is an issue, it will be raised. Be there to support your brother and SIL as they will need you. However if you push for them to intervene before they are ready you may find they push you away and then they will lose your future support. Big hugs, this is not an easy situation.