I know my brother and I don’t have the same father

I (31m) have an older brother (33m) and we were raised as biological siblings with the same mom and dad. When I was in my late teens my mom decided to tell me my brother and I do not share the same father, my dad was extremely mad she told me. My brother was born a year before they even met. I asked her who his father was and she said she “couldn’t remember” - a load of BS but I wasn’t going to push it to get more info because I didn’t ask for the info I had. Shockingly, literally the whole extended family knew and managed to keep the secret all this time. It did make my paternal grandmother(RIP) telling the minister he was going to hell for marrying my parents make more sense if still absolutely insane.

When my brother joined the military my parents explained to him that the reason his name was changed when he was around 1 was because they weren’t married when he was born and mom didn’t have dads name put on the birth certificate but maintained he is the father. What I found out from my parents had decided this was the best way to make sure my brother never felt like he wasn’t part of the family or different.

Fast forward several years and I’ve never told him this because it’s not my place (just like I didn’t tell our parents when I knew he knocked up his girlfriend his senior year of high school). His wife has commented on him not looking much like my dad and I just shrugged it off as nothing and blew past it.

In my late 20’s my parents got divorced. It was a very bitter divorce - mom got very horrible and mean to everyone during the last third of their marriage and my dad probably had an affair at the end with the woman my mom swore for 20 years had been trying to steal her husband… point for mom! Yay dysfunction! I even told my dad I would be surprised if mom didn’t tell my brother out of spite. He said he had thought about that and was ready to take it on if it came down to it. My dad got engaged months after the divorce was finalized and married soon after. I managed to get ahold of my mom’s phone and set Facebook so she couldn’t see pretty much any pictures of the wedding because I knew it would be horrible if she did and she would go nuclear that we had betrayed her. She did find out but I was able to keep the situation contained and avoid the worst outcomes.

At the wedding, my SIL brought up my brother not looking like our dad to my (ex) girlfriend and fortunately I had told her about the situation. She did the same ducking and avoiding I have always done. My brother is starting to get more skeptical of it and asking around. I feel bad for lying but I can’t be the one to tell him even though I know it’s going to be bad if he finds out I knew.

Mom passed away very suddenly last year and never told him the truth and never told anyone to my knowledge who his actual father is. I feel weird carrying it and at this point most of my friends know and agree that it’s something I just need to keep to myself.