I'm going to be brutal about this. You and your family do everything you can to prevent this from happening. However, do it very cautiously. A man in love is not a man you want to frontal force at. Also, you have time on your side. I don't know the values of your family, however he is only 22! Good thing he's not 42.
First, know who your brother is. Is he rebellious? Is he passionate? Is he obedient?
Know who is girlfriend is? Is she a witch? Is she sweet? Is she the jealous type? What are her parents like? Are they for this or against this? If they are against it for the same reasons your family would be, then you have allies that can help.
Pardon me for this, but is your brother a horny toad? Do his eyes wander when pretty girls walk by? Has he ever been single for a long time with lots of girlfriends? I doubt it since he's only 22.
How long has your brother been dating this girl?
I'm 32 and have been married for a year...it's a different world, this marriage thing. There is a time when you are ready and should do it and a time that you really shouldn't do it. It requires a type of maturity that is hard to pinpoint. A marriage can end quickly once the bells and whistles of it become reality. In the beginning it is "Ladeeda"...but once married it's permanent...and then the fights can ensue and who knows...a younger immature man and woman can cheat potentially...
and financially it's very difficult...can't imagine what a 22 year old can pull off. You don't just get married and move into a smelly shack and let love do it's thing. And what if she gets pregnant and they're not ready, then they'll go on welfare like all the Shvartzas..oye!
you have to think about how you are going to approach this...the last thing you want to do is at the attempt of breaking them up, they run away and disappear together.
And for those of you hear you who said she should convert, I say, "no way, Jose!" It doesn't work like that in Judaism.
I have so many questions to ask you about your brother and his girlfriend. P.M. me and we'll talk.