I (21m) am currently living two hours away from my family. I like to visit once or twice a month. My family consists of my mom (51f), dad (49m), twin sister (21f), and my younger sister (11f).

Naturally being the first born children, my parents were a lot stricter with our upbringing, but I feel like it definitely helped me and my twin develop good strategies to regulate our emotions (we are all ADHD kids btw). It was a lot different with my little sister and she becomes very aggressive and belligerent when told no. She yells repeatedly what she wants if told no. Everyone else is done with her which is completely fair because that's their everyday life. I'm trying to talk to her, but she never seems to learn.

In general, my younger sister is really only agreeable if you give her what she wants and becomes rude the moment you try to suggest anything else or just say no. I honestly think she needs some sort of extensive therapy, but is there anything else anybody would recommend?