User blog comment:Tatertot623/Go to M'Gann's page/@comment-75.42.221.175-20111113024606/@comment-432124-20111212053046

I'm thinking that that's the most likely possibility since from what we've seen in "Bereft" and "Secrets", M'gann had only known J'onn for 3 or 4 months before "Bereft", but 6 months before that, she was already calling him "Uncle J'onn", so it's impossible for her not to be his niece. In TV Tropes, one troper had an interesting theory that she was a hybrid martian and that J'onn's evil twin brother is her father. I think that is a possibility.