I think it's a little far fetched to say that Overeem backstabbed his own brother. We will find out overtime though. If he did do that then it's a surprise to me I will tell you.
His Brothers Career was over anyway the second he pissed his pants and taped out because Griggs gave him a dirty look, I mean come on seriously was Zuffa ever going to let him fight for them again after that, seriously I seen people tap out to strikes a little easy before but that was the first time where you could actually argue a strong case that the method of submission used was a dirty look.
Maybe his brother knew he was finished and was never going to get a UFC contract anyway, so maybe the Reem offered to see him right in the name of brother love and all that, seriously think about this next part what has his brother really lost out on, Striekforce is dead and during the rest of its short existence how many more times do you think Valentijn was going to appear on the cards anyway if any.
Maybe he is just happy for his brother to pull in a huge UFC contract that he would never get offered himself, and let his brother see him right so he can retire.