It it possible that he could get in trouble but that would be up to your parents. You are still considered a minor so you cannot move in together without parental consent.
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In Illinois, 16 is the "age of consent." A person who has sex with a kid younger than 16 may be charged with statutory rape. Assuming you two are working hard to take care of the twins and are working well together (no abuse, no drugs, no unemployment, etc.) then it would seem a bad idea for the State to prosecute the children's source of support.
There is a case, Mitt v. Ham, in which a 40 year old man married a 15 y.o. girl and they then lived together. The girl's parents had him charged with abduction and statutory rape and they then locked their daughter up in their house to keep her away from him. He sued her parents to compel them to release his wife (the 15 y.o. girl) . . . and he won. The charges were dropped and they lived happily ever after.
Talk with BF and Mom and Dad about getting married -- it might be the right thing to do.