Most Helpful Guy
2 possibilities, and I'll let you figure out which applies 2 u.
1- Older bro hates your Boyfriend and is showing Boyfriend this hate by dissing u
2- Older bro is an ugly fat guy or computer nerd who has no GF, can't get laid, and is jealous of his bro so much that he tries to stop little bro from having a healthy relationship. This is not unusual in families where one kid is ugly. If your Boyfriend had any earlier GFs, ask him whether Big Bro was a d!ck then. If he says yes, your def. in situation 2.
If little bro had no earlier GFs, then this is a problem between big and little bro. The only thing you can really do is ask little bro to have a chat with big bro about why big bro is dissing u.