What if she was the aggressor/instigator?
You're 20, your a little buzzed, athletic blonde is flirting with you, she initiates the lovin. Hell, do we even know that he knows she's 17? She's a volleyball player who is like 6'0". I don't know about you all, but I didn't check ID's when I was a sophomore in college. The girl is a senior in high school and he's in a sophomore in college. Not a whole lot of age difference there.
As long as she didn't ever say no, push back, tried to stop, tried to get away, implied at any second that she wasn't all in on the deal, then I can't fault the guy. I would be a hypocrite if I did because at age 20 I promise I wasn't checking ID's on girls I met at parties who at least looked old enough.
And as far as being a teammate's sister, I knew plenty of girls in high school who would hook up with a player on the team that their brother was on.