Finish off her boobs

You can’t switch targets now your balls have taken too much damage.

You go for her boobs again and punch each one several times. Your sister doesn’t look pleased and also looks to be hurt. One more boob pounding should do it.

“You had your fun it’s my turn.” Your sister says

She kicks you again square in the balls. But this time one of them did not regain it’s original shape.

“Squick!”

“I think I broke something down there” your sister tells you while jumping in happiness.

You feel your ball. It’s not totally ruined just a small rupture. One more kick and your ruined. But same goes for her tits.