On either side of him, the shape of her legs began to form back out of slime, with her lower legs flat as her knees faced the wall behind him. At first it was just on the ground, before it extended up from each knee, slightly rotated to be more towards his lap.
The slime in his lap engulfed his hand as it went to merge, her crotch and genitals forming around it. He could feel all the slime moving as he attempted to take his hand out, but it appeared stuck... right where her vagina was.
The slime slid up the existing body and continued to merge into the shape of his little sister, completely in the nude, with Jeff noticing he may have made it a bit too sticky...
The torso finished forming, as the head and arms started. Jeff watched as the slime girl finished forming, strands forming her hair at the end. She leaned forward, sticking to him even more as she appeared to be unconscious...
"Uhh- Now what do I do..." He didn't anticipate the changes being strong enough to entirely trap his hand inside her, unable to pull it out, while attempting to push her off to unstick resulted in his hand even further in. The slime girl sat in his lap, entirely unaware of what happened now...