Hi Missrubble and warm welcome to our forums
Life sounds very difficult for you. There is a lot going on so I'm pleased you've found your way here and I am sorry that it's taken so long for you to get a response. But please know that it's not your post, it's just what happens here sometimes, posts get missed.
You've explained yourself very well, there is nothing to apologise for. It sounds like you are caring for everyone, and not doing much for yourself. Do you see anyone about your low self esteem? For example a doctor or health professional? I'm not one, just someone who has PTSD, anxiety and depression.
Being their for someone all the time, like you are for your mum, is very demanding. It's important to care for yourself as well. From my own experience, my mother lavished attention and money on my younger brother, along with leaving the house to him. There were quite a few reasons for this. Many of which I did not know until she had long passed away. It was her guilt of what she herself had done to him. So maybe there are things that have gone on between your brother and you mum that you don't know about which has caused all this attention.
One thing I've found is - not to let that bother me. Mainly because to try to influence what she did - was beyond my control.
I'm not sure how much this helps. Perhaps, you could do keyword searches for - family relationships using the search tool at the top of the page.