The twins are not 10 years old now . Time for them to,grow up and lead their own life's . 
Not liking siblings this unstable.
Lopezs functioning fine on separate teams.
Don't want either Morris on team......whining and moaning
But you're missing the point. He's not upset because Marcus was traded, he's upset because he was betrayed by the Suns. Goran Dragic would say the same thing.
They took a discount to stay together. Markieff could have easily not signed an extension and make more than what he's making right now next year, but they took a massive paycut to stay together, and Phoenix obliged, only to trade Marcus a year later.
Sure, they have to realize at some point that they can't live together. But this is more about betrayal than actually being separated with Marcus. The team he's in stabbed in him the back, while restricting him financially.
Add to it that the owner called him a privileged millennial as to why he's acting out (completely ignoring the fact that his team did this. Anyone would be upset with that.