I'm the younger one of two siblings, my sister is three years older than me. I know it's natural to feel protective over your younger sister but she has to make her own decisions. For a long time I dated someone two years older than me, no one in my family said anything. When that relationship ended, I started dating someone three years older than me. My sister freaked out and called my boyfriend a pedophile (I was 20 and he was 23, so both legally of age). Her overbearing nature really drove a wedge between us, I don't feel that she has any right to comment on my relationship especially when it's so uncalled for.

So, my advice, don't be overbearing or over-insert yourself. You are allowed to have opinions of course, but try to get to know the guy and try to get to know them as a couple. You might be pleasantly surprised.