7 Deadly Brothers: Modify this character's attack value by +1 whenever you control 4 or more characters named "Seven Deadly Brothers." Instead, modify this character's combat values by +1 whenever you control 7 or more characters named "Seven Deadly Brothers."

If both my opponent and I have some Seven Deadly Brothers on our teams, and then one of us manages to control one of the other player's SDBs (say, with Mind Control), can we use this newly accquired SDB to count towards the total number required by this trait?
Sure. Let's give a concrete example:

You control 7 SDBs and your opponent controls 3. Normally, your SDBs would all modify their combat values by +1 (since you have 7) and your opponent would have no modifiers (he only has 3, not the minimum 4). But your opponent also has a Mind Control character on his team.

Your opponent uses Mind Control on one of your SDBs. While the action that activated this use of Mind Control is in the process of resolving, that SDB is now friendly to your opponent's team not friendly to yours. You now control (for the moment) 6 SDBs and your opponent controls 4 of them; until this action resolves, then, you both modify your SDBs attack values by +1 (since you both control at least 4, but neither controls 7 or more). This can be particularly important if the now unfriendly-to-you SDB turns around and attacks an adjacent SDB that is still under your control.

But once actions have completely resolved, and the Mind Controlled SDB returns to its original force, your opponent will once again only control 3 (so no modifiers) and you will control 7, assuming none were KO'd during the Mind Control action (so +1 modifier to all combat values).