This is a tough situation. I am a twinless twin and the first thing I'd recommend holding off judging their relationship as inapprpriate. You are more than entitled to share how you your partners actions make you feel and you can ask for what you need - but it can be hurtful for someone you care about to judge a close relationship with a family member as good or bad ...right or wrong.
I think that many people are unfamiliar with close connections can see this as threatening. His twin will always be his twin and, while he could be so much better about making you feel included when he is with his twin, maybe there is an opportunity to honor his connection with his twin....I bet he will appreciate your support, love, and acceptance of his situation more than you realize.
