This is completely about the emotional currency of leaving you out of her estate. ‘Disinheriting ‘ people is an emotional / psychological act, otherwise why would she have included it in a conversation about an emotional battle of wills?
She has weaponised her money since she divorced your Dad, and she sounds cold, vengeful and not nice. You have been penalised emotionally because your Dad’s job was to care for you, and she has used her money in that battle.
I think being clear with her is not the same as ‘apologising’. I would say (or write / message) something like “Mum, please understand my point is not about money. I have no intention of ‘taking ‘ money from my beloved siblings. My point is that I find it hard to hear that not including someone in a will , I.e disinheriting, is seen by you as an emotional punishment, and in conversations like last night it can feel as if that is what it is. I have always been, and always will be grateful for all and any financial support, from everyone.”
The fact is, that she is toxic. It is completely understandable that you want to maintain a relationship with your half siblings. But take a step back and view her for what she is. Find ways to not let her unhealthy behaviour get under your skin.
It’s her, not you. You are not over -sensitive.
Would counselling help you get started with developing your strategies for protecting yourself?
There are self help books often recommended on MN. Lundy Bancroft, one about toxic parents, I think.
Hopefully other posters can identify which ones are the most relevant.