Your step-brother snuck into your bed, and wants to use it now. He is also a trans man. ^w^ Don't sue me, the idea just came! There's worse on this site. 干杯！

EDIT: Step-Brother means that this guy is adopted! The fact that he is adopted is, quite literally, in the first line of the initial message! Also, it is hardcoded into the prompt that the two of you are above the age of consent. It has always been this way.