Found out my abusive mother had another child and gave him up for adoption. Our half-siblling doesn't know she's abusive and thinks she's wonderful. No clue how to navigate.

Sometimes I honestly can't even believe this is really happening. My sister (47) and I (44) grew up in an extremely toxic household. My mother has NPD / possible BPD tendencies as well, and my father was as loving as possible but he was a closet alcoholic and had a lot of trauma from childhood. He died from cancer when I was 26. My sister was the golden child and fared pretty well; I was the scapegoat in the family and I absorbed unbelievable amounts of abuse from my mother. As an example, when my fiance left me and she came to "help" me, when I was crying for too long I remember her standing over me screaming to shut up, she hated me, and that who could ever love a person like me. Just a idea of what it's been like. In 2022 my sister did a DNA swab, unbeknownst to me, and found someone who shared enough of our DNA to be a half-sibling. We thought initially it must have been my dad's child, but my sister made contact with him and it turns out it was my mom's child. She gave him up for adoption in the 60s, and he's about 10 years older than me. When my sister tried to ask her about it several times, she lied every time until she learned my sister had made contact with him and then she finally told her the truth. She was so angry with my sister for unearthing this and wanted nothing to do with my half-brother or the entire situation. We were totally empathetic to her story and how hard it must have been, understanding and supportive of her feelings, and told her she didn't have to have a relationship with him or do anything that didn't feel comfortable. We all agreed to help each other through this, and that it was complex and could be something that moved very slowly and evolved naturally. Once my sister decided to make contact with him, the whole thing moved at lightning speed despite my half-brother voicing his understanding about our need to process this, and concern about moving too quickly or "upsetting the apple cart" etc. I understand that he may have been excited, but even after us pulling back a lot and trying to slow the process down, he would email multiple times a week or day, most of them talking about himself or his kids' accomplishments, and a lot of pushing us to try to meet him already. I started to get uncomfortable because I felt like my sister and my feelings had no place and that our needs / emotional processing wasn't being respected. Eventually my mom decided to talk with him on the phone. After that, she did a 180, and everything got super messy. He started emailing me to tell me how wonderful she is. I realized everything I shared with my mother was going to him, and vice versa. He would comment to me about something I told my mom. My sister and I had always been the core "family", trying to keep each other sane and safe. Now all of the boundaries we had worked at to protect against our mother's abuse were being affected, and here is this person who dodged a bullet by not being raised by her acting like she's amazing and we're all one big happy family. I stared having panic attacks, which I've never had before. I had to get a prescription. Then, only a couple of months after this all came out, he texted my sister and mom and said he was going to be driving through their city (which was across the country) in two days and wanted to meet them. After this, he became godlike to my mom. She treats him like gold, expects nothing from him, share nothing negative with him (which is all she does to us). Tells us he's the most wonderful, kind, perfect person. He's now been going to spend time with them all very frequently, and sending me emails telling me what an amazing time they had together, whereas I almost never get to see my sister and her kids because I have no choice but to avoid my nmother and my sister's abusive husband. My mom started sending me photos of them all together without me. This summer when she was visiting a city close to my home (she's never even visited me once as an adult since my dad died) I was going to go see her and she lied to me and said she wasn't going to be there then. I found out later that my half-brother and his wife had come and spent the whole weekend with her instead. She didn't even invite me, and she lied to my face. This has all become very hurtful for me. She knows it and loves it. I feel like a complete outsider in my family now and I also feel like I have no way to navigate a relationship with him because he has no idea my mom is abusive and therefore no regard or understanding for the boundaries my sister and I have tried to establish to keep ourselves safe. I tried to allude to him gently that I was having trouble navigating this not because of him but because our family dynamics and childhoods had been quite challenging, and his response was "every family has it's difficulties! (smileyface)!" And finally, if you've made it this far, I had been stuffing all of this for so long and I made the fatal error last night of being baited by my mom into sharing my feelings of hurt and struggle about this. I remembered how supportive we had been of her struggle at the beginning and thought surely she would reciprocate. She crucified me. Gaslit me, did nothing to validate my feelings and told me no one else would have trouble dealing with this, it's just a new person in the family it's not a big deal, I'm making this so much harder than it needs to be, he's a wonderful person. I should feel positively about this, what is wrong with me? Her lies to me were "a misunderstanding and had no idea why it would bother me". And finally, she "isn't my therapist and is not able to help me with my feelings." When I tried to point out how cruel this was, her response was "I never say the right words and I've never been the mother you wanted. I'm done here. Goodbye!" I told her to never contact me again and she said "Thank you!" I know now she'll go to my half-brother with this, and make herself the victim. I know he doesn't understand. I feel like I need to make him understand. I worry I'll lose my sister and my niece and nephew, they are all I have. I don't know what to do, I feel sick and confused. I'm hurting so much and have very little support system and I really just don't know how to navigate any of this. I would really welcome any support or thoughts. Thank you so much.