When you fight a friend over a principle, they don’t see the point, they only see you. When they run out of words to fight over, they are left only with echoes of what was said. And eventually, because words spoken and received emotionally imprint the deepest, they have no choice but to see them for what was truly said. And that is why it is better to be silent than to speak recklessly in anger.
It sounds like you both need to have a heart to heart putting all your issues and fears on the table and then come up with a plan to face them together so that they are no longer obstacles to the future the two of you could so happily have. But you both have to be willing to do it.
Are you being too picky? Who am I to say? We all have a right to say for ourselves what we require to have peace in our lives. Your sibling’s mates may drive you crazy but there is always someone out there who is made to love somebody , even them, and even you.