My wife changes her clothes in front of her twin brother

From a male fan,

My wife's twin brother is currently living with me and my wife but his stay is now causing us problems as my wife keeps changing her clothes in front of him. She's never completely naked in front of him but she has no reservations in taking her top off and talking to him in her bra. She says they're very close, that there is nothing she is hiding from her brother, forgetting she is a married woman. I have tried to make her understand my position but she says I am making a big deal out of nothing. Am I really making a big deal out of this or it this a big deal and something I should be worried about?