I think you are doing so many proactive things - you are looking for work, you are seeing a T every second week, you are seeing your psychiatrist every month, you are going to start volunteering in Jan and you are waiting for a whole host of blood test results - I know you are struggling and having a hard time at the moment, but I see you making so many big steps in the right direction. In reality, though it doesn't feel like it, you are doing really well.
The trying to understand your child abuse is a pretty bad rumination for you to get in. There is no logical, emotional reason for why those kids in your family got treated okay (well apparently to your eyes, but who knows what is going on behind closed doors) at the very least the well treated children saw how the badly treated children (you) in your family got treated and at least on an unconscious level have lived in the fear of ending up being treated like you did.
You will never get the explanation or the understanding from your mother you are seeking. I have sought such things for about 30 years now, some of the time was unconscious and some of it was not. It is not coming for me, and I doubt it will be coming for you, if your mother was able to explain or apologise or give you nuanced understanding and was able to see your point of view, then you wouldn't have been abused.
When children are abused - it is not personal - people are acting - well parents are acting out their demons on their children, they often have not ability to see their children for who they are, and very rarely apologise, understand, give logical explanations or give any real logical reasons for why things were/are the way that they are.
Some people in families and societies are the scapegoat, it enables other people to get rid of their shit and to dump and then get on with their lives, often with very few, if any consequences.
I think you have a lot to offer Jess. I hope things ease up for you soon.
You did not deserve to be abused, you were not personally abused, thought of course if feels tremendously personal. As children, people project their stuff on to us, and we have no or very little input in to how we are treated, viewed or scapegoated.
If you keep obsessing about this jess you will waste a lot of time and energy, at some point you will have to let it go, but as I can't let it go, I have no advice to give you. I still want acknowledge from from family I see on television that are doing good works for disadvantaged peoples/refugees/human rights etc etc, but I will never get it and really that path leads to madness, of a sort, for me anyway.
The time might not be yet for you to let go of this, but you said to me once in chat, you didn't need to work for the money so perhaps you need to move out and get away from your family and the thought cascades (only just learning about this today) of rumination that keep coming each time you see your family. They won't change. Your mother won't change. The only thing now is for you to choose your own direction and find your own way in life.