Getting rid of the competition.
"Step-sister spots something small - on closer look is amazed to find it's her little step-brother who has gotten shrunk and is now tiny and helpless. Sympathetic: how terrifying it must have been to be so small! He could have easily been stepped on by someone!

You realize that this is the perfect opportunity to secretly get rid of your competition. Big step-sis could literally lift up her screaming, pleading, wiggling little step-brother and do the utterly unthinkable: lower him into his own step-sister's big, wet, hungry mouth and swallow him ALIV... Show more