I worked with a woman brought up like this. She was the only girl in the family - a surprise baby quite a bit younger than all her brothers, She had been smothered with love by parents and brothers who protected her and rescued her from every problem in life.
She was in her 40s when I knew her. She was a clever woman, but was under-educated, because as soon as a course became a challenge, she dropped out, she dropped out of a marriage very quickly when it wasn't the fairy tale she expected, and backed away as soon as anything required her, to overcome a problem.
I gave her the chance to do an assertiveness cause, which she did (accepting was easier than saying 'no'). The day after the course finished, the course tutor rang me to express her concerns about her, saying she has got deep and complex problems behind her lack of assertiveness, that a short course could not even scratch.
This is what you are dealing with here, a deep and complex problem, The solution, possibly, starts with encouraging this young man to develop some self-confidence and to contradict his mother when she tries to speak for him. Could getting him 2 tickets for him and a friend (and make it clear, this is NOT his mother) to go to concert to featuring a favourite music group be a way forward? I think you need to present mother and son with a 'fait accompli'. 'Here are the tickets, take a friend your own age', so that there is no 'mother comes too' option. On rethinking, First time find something for you and he to do tgether and then go from there.
I think the OP can do little or nothing about her sister's problems, they are too deep and complex, but she could discretely and very carefully start to encourage her son to become more independent of her.