The homewrecker here isn't your cousin, who is barely an adult and 100% dependent on you and your husband, but is in fact your husband who a) is the one who made the promise of monogamy to you and b) is taking advantage of a young woman who is under his responsibility and who probably felt that saying no would threaten her livelihood/is flattered an older man is interested in her. She's just barely 18. I get that you feel betrayed but why don't you turn those feelings toward your husband who not only cheated on you, but also took advantage of a young woman, barely an adult, who depends on him for a home and food.

Your cousin is barely 18 your husband should have known better considering he’s the adult in this situation. There’s already such a huge power imbalance because of the fact that your cousin is dependent on you and your husband for shelter and food. Your cousin was taken advantage of by your husband. And honestly this sounds so suspect considering the fact that I’m pretty sure your husband had known your cousin before she turned 18 and was kinda just “waiting” for that before he pounced. Your cousin still has her high school mentality, your husband is the adult he should have acted like one.