Brother's disrespecting our mother

I'm 25 and have twin younger brothers who are 15. We lost our mom almost 2 years ago. Our mom has been through a lot and struggled so much with addiction but it's because she had been through so much and told me and the boys everything. She was raped and sex trafficked and was in an abusive relationship before meeting our dad and our dad left our mom because she was too much to deal with. My mom lost custody of the boys 4 years ago for getting back on drugs and had our dad got full custody. My mom still was involved, paid child support, and saw the boys regularly and she was heartbroken when she found out they were calling our step mom, mom when even though our mom isn't perfect, she was a GOOD mom who LOVED us. I went off on my brother's for finding that out. Well I have now been told, even though our mom hasn't even been gone for TWO years yet, that our step mom is adopting the boys. When I was told that I lost it. I yelled at them. Told them they're disrespectful and disrespecting our mom and Aria(step mom) will never be their mom. I also reminded them that our mom almost DIED giving birth to them. She carried them in her stomach for 9 months and almost DIED bringing them into this world for them to do this. One of my brothers said they aren't asking my permission. They're just telling me and whether I like it or no Aria is there mom and I was so distraught and wasn't thinking and just slapped my brother across the face. My dad made me leave. I feel bad for slapping him but after everything our mom has been through in life they're throwing all that away for some women who barely been around 5 years. She's NOT their mom and they are disrespecting our mom. I can't believe my dad is letting them do this. Idk what Aria is telling my siblings to make them think she's worthy of replacing our mom. I just feel like fuck my dad, fuck my brothers, and fuck Aria.