Things I can't figure out

[John_Walden]

1.Brother must have forgoten that Yuna is his cousin I don't know why.

2.Mika must have known about Vegnagun so why didn't he destroy it.

3.Why is it that before fighting Vegnagun everyone splits up to fight a diferant part but Yuna Rikku and Paine end up fighting all three parts with no help.

[Squall of SeeD]

1.Brother must have forgoten that Yuna is his cousin I don't know why.

Because, contrary to popular belief, most people don't care. 20% of all marriages in the world are between first cousins, and over half of the States in the United States allow for marriages between first cousins. Most people don't care in all honesty. It's only in the West that anyone seems to, and that is mainly because as people began to be able to have more freedom in who they chose as their mate, it became an expectation that people wouldn't choose their cousin due to it previously having been a choice that was made for them.

As far as birth defects goes, there's only a slightly higher chance of Western first cousins having children with birth defects than there are with most people. In fact, many things can inrease the chances of there being birth defects, such as marrying outside of one's own race or nationality. Anyway, if two generations of first cousins marry, the chances work out to being even with that of most anyone else.

2.Mika must have known about Vegnagun so why didn't he destroy it.

It's explained in-game that Vegnagun will start going berserk if someone has the intention to harm it. Poking a bomb that could blow up Spira with a lit match isn't a good idea.

3.Why is it that before fighting Vegnagun everyone splits up to fight a diferant part but Yuna Rikku and Paine end up fighting all three parts with no help.

They end up fighting with no help because everyone else was exhausted (story explanation) and because the programmers didn't bother rendering a place in the battle engine for the other characters (game mechanics explanation).

[boys from the dwarf]

here what happens for your third question.YRP do their job then logos ormi and leblanc get battered so YRP does there part for them and nooj gippal and baralai get beaten up so YRP do it for them.

[The Anarchy Angel]

Basically, your characters are more super charged than the weak allies. Though, i think that fighting with other characters with pre set abilities would have been more of a challenge