Your big sister is now huge. Her lower half takes up nearly a third of the couch. Her melon sized tits pale in camparison to the fertile hips that spread across the couch. Another pervy idea slips into your thoughs. You need to see more. Ideas of watching her growing more and more disproportionate fill your mind, becoming more ass than woman.
"Lose those lumps and stuff her rump," you ask the wand.
You watch her twin melons deflate down into two little mounds. Her tiny A cups lost in the folds of her now much looster shirt.
'BLOOMPH'
She wobbles in place as she makes a futile attempt to fight gravity. The weight of her own flesh is too much though and her legs give out. Heavy butt flesh pulls her back down into the couch's embrace. Her lower half surges outward with a few more inches. Her hips are overflowing the center seat, encroaching upon the side cushions. She sits slightly higher from the mountain of padding underneath. A subtle change in posture reveals that she can't even lean back with her gigantic bubble butt taking up the entire seat. You watch your fat ass of a sister squirm as she struggles in vain to fit her big round booty on the sofa, but with cheeks the size of beach balls it was impossible.
"Ugh, this couch is too small!" Peach complains.
Another spell comes to mind.