Dear Penny,
I guess your mother was driven by insane jealousy or some other insane and juvenile emotion - it is clear she is not very well adjusted or domesticated or maternal and none of it is your fault.
I feel it is time to be completely honest about things; your past and so forth to anyone who wishes to know...
You should access any counselling resources your college or university has and make as much use of them as you can.
Clearly your mother enjoys or feels the need to have a slanging match with you: is it just you or is she that way with everyone?
You can simply refuse to discuss or join in. Probably best if you don't speak to her at all unless it is necessary.
I suggest you continue to write about these things. Emptying your head onto paper is great therapy...
If I was in your position I would write to my brother's school explaining everything and send copies to the education authority for the area and those responsible for social care...
I hope now you are older and more mature and have the support of your boyfriend you have made your father fully aware of the facts of your growing up?
Your brother too needs some support: he needs to be reminded of the sacrifices you made, how you are sorry they weren't sufficient but if he doesn't now do his best it will be a personal disaster to you and mean his mother has won somehow...
You have my best wishes!