Hi there and thank you for your post.
Firstly you are not being selfish for wanting a mum who functions normally in your life. One who you can look up to, who can support you, who can guide you, who can model appropriate adult behaviours. It is great that you are wanting her to come up to your level of behaviour instead of thinking that in order to have a relationship with her you need to “go down” to where she is i.e. underfuntioning.
I have no idea why your mum is behaving in that way. There are many possibilities from unresolved issues within her own family of origin ( which may be important here as it started after her mum’s passing) ,to personality styles or disorders, to mental health problems. I just don't know.
What I have worked out over the years is that it is almost impossible to make someone change if they dont want to. If your mum was doing all those things and miserable about it and begging for you guys to help her change , then you have an “in”- something to work with.
However , if someone doesn’t think they have a problem, or else can see they have an issue but doesn’t want to change, then you are really kind of powerless to make them do anything.
So.. where does that leave you ?
I think it leaves you in grief - Grieving the mum that you thought you were going to have, grieving the relationship that you once had, grieving the vision of the family you thought you could have and enjoy.
It’s sad but it seems that you might have to let that go and come to terms with the reality of a new family. That includes a brother and dad that hopefully you can build a good relationship with. But it also includes a mother that may never be kind, caring and supportive of you. And it's not your fault . It is because she is unwell in some way and cannot extend what she should to her child and partner.
So, I say this with much sadness , but I think the best course for you is to let go of the hope of fixing your mum and if you want her in your life, then lower your expectations. Do not expect the “normal" mum that you can talk to properly as that hope will just keep getting smashed and it hurts every time. When she is nice, see it as a rare & pleasant surprise.
Save yourself emotionally. Its like when someone is drowning , you need to make sure you have a life vest on first before trying to go over to them , or else they will pull you under the waves. Get your life vest on… Get your education, develop your friendships, develop your hobbies and interests, develop your family relationships that are going to be rewarding. Be kind to yourself , be proud of yourself and be strong.
When you are at your best my hope is that whatever behaviours your mum dishes up, you will be able to be compassionate to her, see it as “her stuff”, nothing to do with you and disengage without being hurt by it.