If I run a 300 point team of :

2x Osiris
1x Isis

Does Isis get the +1 to her stats two times if both Osiris die? The power doesn't state once per game or anything.

I do not believe so.

No, it only applies once. The condition you check is "has a character named Osiris been KO'ed?" If so, modify Isis's combat values by +1 anytime they would be checked.
If it said "For each friendly character named Osiris has been KO'd this game, modify Isis's combat values by +1 for the rest of the game." then you would get the bonus multiple times.

I figured the wording was meant to work around multiple instances, thanks guys.

What about the other way? If you're running 2 Isis, and Osiris is KO, do both Isis get the +1?

"A SISTER'S GRIEF: If a friendly character named Osiris has been KO'ed this game, modify Isis' combat values by +1 for the rest of the game."

The plus +1 is a game effect, but is not activated by a free action. The limitation of a game effect only being activated once, is only covered under free actions.
I'd say both can use it, and I would have been sure about it a month ago. But now, not so sure.

Absolutely.

That rule has nothing to do with situations like this. That rule is there to tell you that your Mystique can't, say, use Perplex twice in the same turn (barring some corner cases). If you have two Mystiques, they can both use Perplex during the same turn.