Family Ties

Mundungus Fletcher. We really don't know that he doesn't have a wife, even, though that's not too likely (she probably would've been at an Order meeting). But a brother or sister who either considers him or herself to be far more honorable than dear Dung? Or a sibling who is actually partly why Dung hasn't landed himself in Azkaban? An embarrassed daughter (who is out of Hogwarts)? It could be quite interesting.

Ted Tonks. There is no reason he couldn't have any siblings - and I think it would be interesting to see Tonks and muggle cousin. The only possible pitfall I can see here is that Tonks didn't seem to know much about muggles, judging from OoTP. (she's fascinated with the Dursleys and says something to the effect of "I didn't know muggles were neat freaks, Dad's a right slob,"). Therefore, the author would have to work carefully with the latter. Same goes with Tonks being close to muggle grandparents who are still living - then why doesn't she seem to be that familiar with the muggle world?

Kingsley Shacklebot. There is no evidence that he does not have any relatives, although as he has been at the Order HQ more than once on a social basis, there is little chance that he is married. But any other relative works.

Remus Lupin. Given his reaction to Tonks in OoTP, it is highly unlikely he is a "first war widow", and I think it would have been mentioned if he had a child Harry's age. But as far as siblings and parents go, we have no evidence he doesn't have them.

Severus Snape. I sincerely feel that if he does have any family members, the relationship must be written in character. Snape isn't going to get teary over a Christmas present, and sadly, I doubt he's ever said "I love you" past the age of five. Having Eileen Prince still be alive works, but it's going to be a complex mother-son relationship.

Albus Dumbledore. This one is tricky, because it's mixed. On one hand, you'd think any living relative of Dumbledore's would be known about. On the other, nobody has pointed out who Aberforth is, and Aberforth is never mentioned when Dumbledore is ridiculed for his other ties (ie Remus and Moody). Is it because Dumbledore's less famous than we think? Or is it because Aberforth is a recluse? Or is it a flint? It's tricky, and when adding any other loved ones, this must be kept in mind. It's also worth noting that Dumbledore is somewhat distant.

Ron. We already heard of an Aunt Muriel, and it's possible he has more relatives. But once again, this has to be worked with. Why, if he does, do we never see or hear of them? As far as the "seventh brother/sister goes", it's possible (after all, Ron has never mentioned that at least two close relatives died because of Death Eaters), but not too likely. I think this, at least, would have been mentioned at some point. Wouldn't Molly have mentioned losing a child?

Hermione. Grandparents, cousins, and such? That works. But she's an only child.

Harry. By now, we would know. The only way this could be possible is if Lily had miscarried, and still, I think Sirius or Remus would've told him. And if he had a living sibling out there, Dumbledore would have mentioned it. Wouldn't he say, "and by the way Harry, while you're Horcrux Hunting, keep an eye out for a girl with red hair and green eyes?" Finally, JKR has made it clear that all close relatives of Harry's are dead. I suppose one could work with an extremely distant relative (I'm sure he must have some), but that's all.

Sirius. We've met all his relatives (except Andromeda), and we know he was never married. Distant kin is probable, but I doubt he has a sister or a brother who simply never appeared on the tree.

Upstaging. This goes for any OC - they should not upstage the canon role of a character. If you want to have an OC think Dumbledore isn't always right, that's fine - plenty of people don't think he's always right. But don't have that Dumbledore eats "Mudbloods" for dinner. He has faults, but one of his strong points is muggle tolerance. Don't take that away.

Implausibility. There is no way that Harry's twin sister crawled out of the rubble and was found by a lovely American couple. Wouldn't Hagrid have mentioned not being able to find Harriet? Wouldn't Sirius have accused Peter of killing Harriett too?

Parallels that are a tad too ironic. (I.e. Sirius's daughter being raised by his wife's sister and husband...)