Struggling with Cutting Ties - What About My Disabled Brother?

Assalamu alaikum. I’m in a really hard spot and I don’t know what the right choice is. At the start of this year I decided to stop talking to my mother (American/Black) and my stepfather (Saudi/Pakistani). They call themselves Muslim but don’t practice - celebrate Christmas, drink, don’t pray. My mother was born Christian and became Muslim later. My stepfather raised us and converted the family. Lately my mother keeps bringing up hurtful things from my past - stuff from my teen years and my marriage to my ex-husband. I’ve been healing from those wounds for 5–6 years, alhamdulillah, but she recently got angry because I didn’t say “Merry Christmas.” I wear hijab, pray five times, and try to live as a practicing Muslim. As a child I had no choice but to join in those celebrations; now as an adult she can’t understand why I won’t say it just because “it’s important to her.” The argument blew up and my mother accused my husband (Palestinian) of making me abandon my American/Black side and “overdoing religion.” My stepfather said I was “becoming super Arab.” In the heat of it he compared me to my abusive biological father - a man who was never part of my life. My mother said nothing to defend me. I hung up and later texted her that his words broke me and that I could never forgive him for saying that. She didn’t reply. A couple days later she posted a photo of my stepfather online with the caption “Nothing you do is unforgivable.” It felt like she was publicly choosing him over me. My husband, who usually stays out of my family mess, saw how crushed I was. Considering my stepfather’s past - not coming to my wedding, never legally adopting me, kicking me out at 18, and doing inappropriate things when I was a child - he told me I shouldn’t have contact with them anymore, especially since we’re trying to conceive and the stress is affecting me. I agree. They don’t respect me, they enable haram behavior, and they keep reopening old wounds. But here’s the dilemma: my eldest brother’s birthday is January 9. He lives with them and has Down syndrome. He’s my heart. I’ve never missed his birthday or failed to call him. My mother has told my other siblings not to contact me, so I can’t reach them. I feel completely torn. I’ve cut my stepfather out and want no contact with my mother, but my brother means everything to me. Should I call just to speak to him, or completely cut ties? I’m worried about how Allah will judge my choice. I plan to pray istikhara tomorrow, inshaAllah. Please make dua for me.