Horny stepsiblings can't stop sneaking around! Step brother and sister don't mind sharing in naughty threesomes, just don't get caught by mom and dad!. Amateur Step Siblings Caught Sex in HD! Than their Dad and Mom leave a home Step Sister and Step Brother starting to Fuck each other in their Teen Holes!. A stepsibling or stepsib is the offspring of one's stepparent. This would constitute a stepsister (or stepsis) and a stepbrother (or stepbro). Culture. In fairy tales, stepsiblings and half-siblings can but need not take after their mother. In Cinderella, the ugly sisters are the main character's stepsisters.