The work is between you and your boyfriend. Keep talking to him about this difficulty of what to do. If you are hoping for a long-term relationship then talking about what to do jointly about a family member is an important conversation to continue. Talk with him about the best way to proceed — and the options. The goal here is for your relationship with your boyfriend to evolve. You want to keep that as a primary goal. It is likely that his sister is going to have to learn things on her own — and now that she is 18 there may be little her parents can do. Without you and your boyfriend being on the same page your actions may come back to hurt your relationship.