This sums up everything.Luffy is the grandson of Monkey D. Garp. Ace's father (before he died) told Garp to look after him. Garp adopted Ace. After Luffy met Shanks he went to live in the mountains with a group of Garps friends who fosted Ace. And thus they became brothers by sharing a cup of sake.
That's a brief summary.
No it doesn't. If you share a cup of sake it does. Not if you just get some coke and drink it.duh... drink together means ur blood bros