There's nothing wrong with that if he doesn't have a problem with it. My best friend, Tina had three older brothers. When we were freshmen, she had a sophmore, junior, and senior brother. We all hung out together (and still do) and have mutual friends. The only bad thing is that I think every single one of us friends wound up dating every single one of her brothers. That got a little old for Tina. And her mom had to put a stop to us spending the night.