He had to think fast, and realized just how screwed he might've been. He was left alone with only his seven year old sister, Maria, for the week. His parents weren't able to bring her with, so they had to leave Jeff behind to watch over her, and now she was his only hope.
As he was slowly dissolving into a pool of semen, he had to move fast. He tried to think, where would she be. He went under his door, now in a state where he fit, and tried to find her. He quickly poked his 'head' into her room, not finding her there, and went into the living room.
There, on the floor sat Maria, watching cartoons. Most of his body had melted by this point, and he dashed over at the highest speed he could muster, and dove into the back of her pants. Despite desperately not wanting to, he penetrated her vagina and went up into her, knowing it was the only option he had, with the young girl screaming as she felt him go in.
He pooled up in her womb, and having a larger mass her belly stretched, almost ripping her shirt. Maria had her hands in her pants, clutching her privates as she tried to handle the pain of having such a large amount of semen inside of her. He completely finished dissolving inside her womb, and if there was a world record for it, she'd have probably gotten it.
His senses were still mostly intact in there, though the only feeling he had was the warmth of his sister's extremely stretched womb. There were a few drops that dripped out, caught by her underwear. He was certainly glad he limited his time limit to 24 hours, as he wasn't sure if Maria would survive the rest of the week without him.
He could hear her in pain, crying loudly as she tried to walk, calling for him. But she wouldn't be able to find him, for he's right there, a pool of semen stretching her womb to it's absolute limits. He didn't even want to think of what could've happened if she was older. He also couldn't shake the realization that he technically took her virginity. Jeff was in a state of panic still though, trying to calm down as he was now somewhere safe...