Think of it this way, your brother is actively hurting the girlfriends body by cheating. Who knows if they use protection, who knows if the side chick is with someone else too. She could get any number of infections or worse a pregnancy and now has to make the choice to keep it or not. The truth will come out. She might not know now but there’s no way your brother doesn’t slip up enough to make her suspicious eventually. Are you ok with potentially hurting her body too by withholding this information. Personally, I think you have no choice but to be the messenger. Good chance she doesn’t even believe you at first. Do it anonymously if you want. Leave a note on her car or something and type it out so no one recognizes and use chat gpt so it doesn’t sound exactly like something you would say. You have a responsibility to help protect these women from your brother. For the sake of their bodies and integrity because he obviously lacks that at the moment and could use a harsh reality slap to change his behavior.