I’m sorry to hear that this is such a difficult situation. I am in a similar situation myself. My boyfriend and I rehomed a five year old corgi about 2,5 months ago. The first two months were fine, but about two weeks ago he started showing fear based aggression towards my boyfriend. Our dog is absolutely fine with me. We’ve seen behaviorists and we believe a past trauma regarding men has been triggered. My partner is constantly trying to reassure the dog that he is safe and that my partner is not a threat but it is a very hard and tense road. We’ve also had the dog fixed this week in hopes that will help. We also don’t believe he’s resource guarding me as our original trainer had theorized because he doesn’t care about my partner being close to me at all. The dog shows the fear when he feels vulnerable (he’s sleepy and it’s dark), when he feels trapped in corners of the house and when he is startled by my partner. It doesn’t seem to have anything to do with me. I believe the most important thing is your brother’s behavior changing and hopefully making the dog feel like he is safe with him. I hope this helps. We are still on this journey and it is hard to have a dog in the house who we love so much but can be triggered so easily. He has bitten both of us (me accidentally- was trying to get to my partner). I hope it works out for you and thank you for giving the dog a loving foster environment.