I was talking this over with my friend and we wondered who would legally be Sirius' heir, ignoring the fact that he left everything he had to Harry. After much argument we found that we couldn't decide between Teddy Lupin and Draco Malfoy, assuming that the order of inheritance favours all males above females.
Personally I believe it is Teddy, as Bellatrix, being the oldest sister (as listed in age order, I assume, on the Black Family Tree) had no children so it would pass to Andromeda's line (even though she was disowned, it obviously doesn't matter since Sirius inherited Gimmauld Place even though he was disowned). Travelling through Andromeda's line, Teddy is the first male so would therefore be the heir.
My friend disagrees and states that since Draco is older than Teddy, he is the heir, even though Narcissa is younger than Andromeda. Especially since Sirius died before Teddy was born.