You might want to put a little bit more thought into whether or not this is something you can even help with. You are after all his little brother, and relationship problems tend to be quite complicated. If it's a trust issue, that's something that can only be properly sorted out between your brother and his girlfriend.
Without understanding exactly why there is a problem, it would be impossible for you to be able to help, because you do not know what kind of help to give. The best thing to do in this situation is tell him that you're concerned, and that you'd like to help if you can... just make him feel like you're there for him, and that you'll stand by him if need be. That's really all you can do, unless you do more evesdropping, which I don't think your bro will really appreciate.
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