I agree. In your bf’s eyes he did his best (so did his dad) and then asked one thing of you. You went against that so to him, you didn’t respect his opinion.
What will have this achieved anyway? This guy is going to be your BIL so do you really expect your Uncle and him can stop being friends or should? Making relationships sour before marriage is really not the best course of action.
Better to let him (brother) be in the wrong and your future husband and future FIL to know he’s in the wrong. They would always support you within that family unit.
Now you’ve made yourself look bad too by making your bf’s family look bad in the eyes of your own family (your uncle).
Sometimes you just have to know when to let things go.