I doubt Seymour's bond with his mother was strong enough to topple Sin.

As for the Dark Aeons, gameplay-story-segregation. But if I had to theorize I'd say the Dark Aeons are higher beings that just lend the summoners of Bevelle their strength because they're actually on Sin's (Yu Yevon's) side. So they wouldn't even fight Sin, and even if they did, Sin would still win, I'd wager.