Let the owner trading Graham explain his reasoning...
This.
Ask the Graham owner why he feels this trade is to his benefit. Then ask him why he feels this is the best value that he could get for the players he traded.
Then go from there. The measure I would use is do I think a reasonable person could believe as he says he does. Not "do I think he's correct". Reasonable people believe a lot of things that are wrong.
If after that your league believes it is collusion, then deal with it (fining them cap room or keeper slots, kicking them out, whatever is appropriate). If it isn't collusion, then let the trade stand.
A middle ground position if not collusion and if the owner getting Graham is willing to help end the strife in the league, is roll back the trade with his agreement and let the Graham owner pursue another trade for him with everyone including his brother knowing he is available.
By the way, as a possible way of helping avoid this... my dynasty/salary cap league has a Fire Sale rule. If anyone is making trades that might be deemed a Fire Sale, they have to notify the entire league in advance so everyone can make offers on their players and any trades that go down will be at true market value. Our rule lists some of the factors that may cause a trade to be deemed as such (timing in season, current players for future considerations, team is not likely to make playoffs, aging vets for young players, etc), and says they can ask the commish in advance whether something might be deemed a Fire Sale if they aren't sure. And are advised that if they don't check, to err on the side that a trade will be considered one.
After all, talking to more than one team about their players is never going to be to their detriment, so there is no reason not to do it.