I'm a longtime fan who has only become part of any online discussion of late. I am sure that there must be tons of fanfic trying to fill in the gaps.

Certainly, though, there is every reason to think that Susan is really the Doctor's kin. Or at least nothing that contradicts that either during her brief run on the show at the very beginning, or in anything we hear later.

I do wonder what happened to her, though. (Another sad story waiting to be told, no doubt.)