Have you and your boyfriend talked directly face to face with S and told him what your observations are?

That would be my suggestion. Make sure you have told him directly. If he ever asks for ideas on how to get free of his girlfriend, you all can help with that. If he is in danger, call in whatever help needed.

Can your boyfriend get help for himself from a therapist? That could help him cope with having a twin go through all this.

For you, you will have to be extra vigilant in taking good care of yourself. It will be too exhausting and nonproductive to lose lots of sleep hearing story after story about what S is going through. Being used as a sounding board too often won't serve anyone.

If you and your boyfriend are getting too worked up too often you all need to find ways to emotionally stabilize. Healthy distractions and healthy habits will help a lot.

Remember: you don't have to be your boyfriend's therapist, you aren't getting paid for that, it isn't your job. He can get help from a therapist, pastor, etc. to help him cope with all this. S can also.

Good luck!