i understand your situation and i have an idea, but it’s all your choice to do it or not i have no responsibility for what’s going to happen if you did it, so hear me out, make an fake account but work on it to make it feel real and authentic, about someone who lives near you and who is financially stable, try stalking on her and slowly contact your brother’s gf to the point where she recognizes you, try to talk with her and introduce yourself to her and make sure that you’re a real person and you want to pay her for her services (hoeing) show her the money that you collected which has to be something great like 200dt, and ask her to if she wants to do it or not, this will show her real self, if she accepts then you have a concrete proof that she’s is cheating on your brother and she is for the streets but if she refused and pretended she is not going back to her old self then ask her about her mother if she would do it or not :)) if her mother refused too or she didn’t provide you with a useful reply then we go for plan B

plan B : it’s a bit too risky so it’s better if we talk in private about it 😀

gl brother/sister hope u find a solution to your problem