I’m so very sorry that this situation is hurting you so much. When a family has been close, it is especially hard to go through tough times.
I think the situation with your brother is different than with your sisters. Your brother is in a terrible situation. His wife is asking him to choose between her and his family. No one should ever be asked to choose between the people he loves. I don’t know why she wants to separate him from his parents and sisters. Sometimes this happens because the wife is threatened by the closeness of the original family and wants her husband all to herself. She doesn’t realize that your family could become an important support for her. And she doesn’t realize that often a husband in this situation eventually resents having to make the choice.
In the situation with your brother, my suggestion to you is that you keep an open heart and mind and be as friendly as you can be to him and his wife when you do see them. Usually this kind of situation eventually settles down – as long as people don’t make a big issue of it. At some point, your brother is probably going to feel secure enough with his wife that he will let her know that he misses his family.
With your sisters: I think something different is going on. Your younger sister is at a developmental stage where she is trying to find her own identity. She needs to figure things out for herself and make her own decisions. At this stage, she needs you to listen sympathetically and to let her know that you think she can make the right decisions. She doesn’t want advice from you. She only wants your support. If you try to tell her what to do, she will push you away. It doesn’t matter if you are right. As difficult (and as unfair) as it might be, the best thing you can do now is apologize to her for trying to interfere and then let her know you have confidence in her. Mean it. Then just tell her you miss her. Invite her to spend time with you now and then but don’t tell her what to do. Since you got along in the past, I’m betting that she will eventually come back to you.
Your father is right that it helps to have patience. I wonder if he and your mom don’t intervene on your behalf because they think it will make things worse. It might. They may believe that they can count on the good upbringing and closeness you all had to be a powerful enough “glue” that everyone will get back together some day. They have faith that with patience from them and more maturity in all of you, things will improve. They may be right. Give it a chance.
Meanwhile, you need to get back to the business of your own life. Work on your studies. Enjoy your friends. Be on the lookout for that someone special. As the older sister, you are an important role model for your younger sisters. If they see you as managing your own life and being happy, they will be much more interested in you.
I realize that the situation is hard for you. I don’t blame you a bit for being sad. But I do think that much of this will resolve itself as you all establish yourselves as young adults.
I wish you well.