Two timing questions, first...

Thorcules is attacked while on Impervious. They're hit for 1 damage past Imperv, putting him on his next Imperv. Then they're hit again for 1 damage, putting him on a click with "My Brother" and 17 defense. What's his defense value for a third attack? Are the first two attacks counted, even though this power wasn't showing when they happened, or started? Is he now at 19 defense? Or 18? Or 17?

For the second question, let's assume it's a fresh turn at "My Brother" is showing. When does the +1 kick in? As soon as the attack is declared, or after the attack is resolved? I would think that it's after the attack is resolved, so the next attack would be against his defense +1, but I'd like to be sure.

I do not think timing matters with this power. The power says IF they have been attacked this turn modify by +1 per hit, so in your example they have been attacked twice already so they have a +2.

that's how i'd see it...

It's worth noting that they didn't necessarily have to take damage from an attack for it to increase the defense. The attack didn't even have to hit.

count how many prior attacks against them. they get that much more defense

they haven't been attacked until the attack actually occurs, so they don't get the bonus until the second time they are attacked in a turn.

The third attack, you check for "My Brother" when the attack is declared, because that is the first time it's been showing that turn. Two is the number of times they were previously attacked that turn, so They would get +2 defense.

I'm gonna team up Thorcules with AN Doomsday, for a "hit me if you dare" team! Of course, it's only for a single turn, so it's not the greatest defense. But it could be cool