Ha! I saw this thread just as my little brother called me...He's 19, went through an intense sex ed course and knew this stuff (he's particularly concerned about paternity and child support. FWIW: He feels like if she's been tested for STIs and they have an agreement, the decision to trust her is his.) We've also always been able to talk about sex, so I asked him what he thought.
He said he could think of lots of arguments, but he could also hear himself, at 17 thinking he was doing the most responsible thing, responding to all of those arguments, "*&$^@ you!", at least in his head.
We came up with two possible approaches that would ease past the "I know everything I need to know and am making my own darn choices" wall:
(1) If your brother has NOT had the kind of sex ed course my brother had, open with, "you know, I didn't know this when I was a teen, but the sex ed courses in schools suck because of some really unpleasant political manuvering by folks who don't think anyone should have sex before marriage and I know our parents didn't give *me* all the information I now wish I had. I've been thinking about you and thought maybe you'd let me tell you what I wish now school or mom and dad had told me." Then tell him about bcp failing and paternity and stuff, just in a "just the facts, ma'am" kind of way.
(2) Tell him you've learned that different birth control pills have different effectivenesses and you're hoping his gf is on a good one, but you've been worrying 'cause you don't want to see him tied down with child support. Then ask him if he'd be willing to find out what she's on and tell you and then you'd research it. Then you can come back and say, it's only whatever % effective, FYI.
That's what my 19yo brother thinks *might* work, anyway
FWIW, I think as an older sibling you have a unique position that works--at least it works for me. It works if you can fairly consistently treat him as an adult, responsible for his own decisions, and not like a kid. That opens a different kind of relationship than probably any other adult in his life. It's just hard to give advice from that position, but when you can give him new information and just kind of be there, the results can be really powerful. My brother and I have talked A LOT about sex, we started having the kinds of joking conversations you don't have with your parents when he was 14...and now he's our donor, so...you know, more detail than most siblings talk about.
