Go ahead, I have broad shoulders, for a girl.
I would get a dolphin costume and demand everyone call me Flipper from now on. Or whatever your favorite animal is.
Hey if she can play make believe and everyone has to play, well how can they deny other peoples demands?
Fine. You have broad shoulders. Go in and blow your SIL’s relationship up. Call your brother a cuck to his face. Go in fearless. Light the harridans ass up. Obviously everybody in your family fears her wrath. Show her what wrath looks like. 13 yr olds have no clue about sex and gender. The woman is a moron. Your extended family will either stand with you or wont. If they do not, to hell with them.