Look at it this way. When you're with your girlfriend, do you want to be alone with her ? Age is but a number and shouldn't be a restriction. Yes, at our age 3 years might seem like a lot; however I'm sure that a good amount of your parents have an age difference. Just because he's older, doesn't mean that he thinks with his penis. He still might have some moral decency left and won't get into your sisters pants. You're being a good brother by looking over for your sister, but you're overreacting.
