I'm going to write off my cousin for buying a truck from my ex
Why does it matter if your cousin bought a truck from your ex?
It is my ex and he should be supporting me and not them.
I can understand your feelings about the situation. It is wrong to not to take your family's side in a dispute. However, just because they are buying something from him, doesn't mean they don't care about you.
He should remember that I'm family, I gave him the idea of making money from selling cars (we work at the dealership together) and he owes me. I'm losing money and he's profiting off it. It's just another way he's hurt me.
I know it might seem like a betrayal but you should give your cousin a chance to explain before writing them off. Family are expected to listen to one another before making big decisions that affect everyone.