OK, I've put forward the idea of more time with dad, but I can understand you do not want to wreck your mother's life. I think she is lucky to have a daughter who tries so hard for her.
I'm no doctor, apart from a MH condition I was faced with a very reluctant teenage girl when I remarried. It never worked properly, though we are civil, as she has always felt her mum betrayed her dad by seeking another, after he died young.
So I've no real answers. I agree your mother needs love and affection, your dad has found it, and you are stuck.I guess for your mother to allow him to go and weeps alone shows how much she will do for you - which might make you feel guilty but is in fact a wonderful thing. She herself no doubt feels guilt too.
May I ask some questions? No need to answer, though you might think about them, probably hard not to.
Has anyone such as a boyfriend of your mum mistreated anyone, you, your brother - or your mum? Or tried to lay down the law?
Second what goes though your mind when you see the boyfriend arrive? What do you think is so upsetting?
The second is the hardest, it takes in so many things, the absence of your dad in the house, lack of support as both mum and dad now have more priorities? Trying to find something wrong? The thought of your mother being intimate if the bf stays over? Plus more.
Do you think the bf would like a genuine friendship with you? Nothing overwhelming, just an improvement until each is relaxed with the other.
Those are just some thoughts, maybe wildly wrong -I apologize if so.
When you think about it you have a lot of influence - or power if you like. And you want to use it for good, let your mum have a happy life, a caring loving attitude, as I said, she is lucky.
Another approach: is there anything at all you might have in common with the bf? Music, books, movies, sport, ... Even in tears reaching out for a moment might help.
No answers as you can see, and maybe a councilor will help in time, here you are cared for and will at least have us be beside you even if I am full of silly sugestions