I am a 27 year old and my cousin she is 15 but not blood cousin i just claim her as one and so is her boyfriend. I like them but I don't want to get in trouble since they are under age. What do I do since they both like me?
You are in your late 20s and these people are minors. You also see each other as family. Together these facts indicate that it would NOT be safe, healthy, or even legal for you to enter into a sexual or romantic relationship with these people.
You are the older adult, so you need to manage this situation with responsibility. Be clear with these two young people that while you support them having healthy, happy, and fulfilling polyamorous relationships with others, you will not be engaging in that sort of relationship with them because it is simply too high risk for everyone involved.
Set firm boundaries and, if necessary, take distance from them until this crush passes.