Mamma66,
I doubt if you would get anywhere by trying to talk to her. She is unreasonable. She would be unreasonable and upset you even more. Hold on to what your younger brother said. Be sure that he is not the only one who thinks this.
I am so sorry she cuts you out. I am sure she is jealous of you. There are quite a lot of people like this in the world and we will all probably come across them in one place or another. I expect she is horrible to your brother behind closed doors. The best you can do is to show him you are there for him and to ignore her as far as is humanly possible.
I am truly sorry she hurt you by cutting you out of the girls' outing. I don't know if one of the other members of the group could keep you informed about any future events so you can get round her attempts to not inform you.
Being inexplicably hurt and cruelly treated by somebody, often in the family, is a common distressing problem brought to the forum of Gransnet. It shows how many of these cruel people exist. We are almost all likely to bump into one of them in some sphere of our life at some time. Such people will not change and will not be reasonable. It might be that they had an unhappy childhood, as you said, but research suggests that more people from unhappy childhoods do not behave like them. they choose to be nasty and are responsible for their behaviour. They get a kick out of it, as simple as that. Trying to reason with them is therefore completely pointless and only leads to more pain for you. They will turn anything you say against you and probably tell lies too. Avoiding them and learning how to protect yourself from their nastiness and not let it hurt you is the best way forward.
They will have been noticed by others in their evil deeds. Many people, scared of falling foul of their bullying, will suck up to them and try to cover up for them. They will become part of their gang, as with the gang of bullies in the school playground. Ignore those people. Just remain serene and do not engage in any kind of ''discussion" with the the nasty person or with those who are supporting her.
Good luck. I expect she is keeping you away from your brother, whom she is cutting off from his family. Make sure you keep in touch with him. Perhaps you can see him at a time and place away from her, maybe in his lunch break? He will need you one day. Right now he may still be trying to understand her and be embarrassed to admit she is horrible. Don't bring up her nastiness with him but let him talk about her if he needs to.
Wishing you all the best, lots of love, Elle x.
I've got another 'keen'... Ouch!