(Please click on the above names to see their bios!)

Note on the family tree:
Some of the relationships are guesswork, particularly on Molly's side of the family. Although it has not been specifically stated that Septimus is Arthur's father, he is the only Weasley listed on the Black Family Tree and according to Sirius, he and Arthur are "something like second cousins".

The symbols represent the following relationships:
= (equal sign): marriage
— (solid line connecting people): siblings
| (solid vertical line coming from equal sign): children from the marriage
{ (bracket): general relation, related somewhere in that particular branch
? unknown name

The Black Family Tree from HP Lexicon helped me in gathering some information as well.
Additional info was taken from the epilogue to The Deathly Hallows and this web chat with JK Rowling.