Is it okay that your boyfriend/husband washes his clothes with his female cousin?

My boyfriend moved to OH because he got the job there and we were talking about moving there soon. Ever since he moved to Ohio and stayed with his cousin, he has been giving his cousin's daughter more attention than me. He looks at her and plays with her like he does to me. She's flirty towards him and they both act like a couple but except they don't kiss or anything like that, that I know or. He's 25 and she's 16 and I'm 20.
Am I going nuts here?

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