As to why this flashback was introduced now and not before is probably because usually a flashback like this is a sign that someone involved will die. And Ace dying came out of nowhere. Some people thought it might happen but i don't think many continued to think that way when it started to become clear that Whitebeard would sacrifice himself. Usually when someone of his stature sacrifice himself it means the person he does it for gets to live. So i think it was great that they showed it after everything was over.

And the flashback itself is brilliant too. Not only does it show how they became brothers and the hardships that made them the strong fighters they are today, but also introduces a new interesting character that hasn't been seen before. And unless Oda kills him of during this flashback it's pretty safe to assume he will show up sooner or later.

And for Sado himself. He was actually close to Ace's skill and if he progressed at the same rate as Ace and Luffy that would mean that he is somewhere around their level and will be a force to be reckoned with.

This is definitely one of the best flashbacks i have seen.