Bellatrix is a cousin. Narcissa Malfoy is her sister, so also Sirius' cousin. And the third sister, I think, is Andromeda, who was Tonks' mother (and Sirius' favorite cousin). Per the tapestry, I believe Regulus was Sirius' only sibling. I think Dumbledore refers to that fact as well when discussing Harry's inheritance from Sirius; at least I do recall he mentions Regulus being childless.
I have to go back and re-read this whole thread; I am really enjoying it. Has anyone else wondered about Lily Evans' history? I've always wondered about her having red hair, like the Weasleys (of course, if they're related, bad news for Harry and Ginny!).
Finally, if Regulus did take the locket, who was with him when he did it? Wouldn't it have to be someone who was gifted enough to re-make that potion? I wondered about that, too, given Slughorn's rhapsodies about Lily's amazing potion making abilities.
What about Lily? She was good at potions, after all.
I've just reread your post, now I realize that is exactly what you were saying, regarding Lily, not Sluhorn. Sheesh! I'm tired!!!