He thinks on it for a bit, before making a decision. "Hmmm... How about keeping it simple." He starts getting to work, typing out the changes he wants.
"In this world, everyone with a younger sibling will live inside of the sibling directly after them. In the case of twins, this is decided by the order they come out. Those inside can still move out of their sibling; If the one moving still has an older sibling who hasn't moved, said sibling stays inside for the move."
"The youngest sibling will live in the house with their parents, housing their various siblings inside. If old enough to do so, he or she can manually force whoever is inside of them out; However, if someone is inside the one directly inside them, there is no control over that one."
"Anyone can, technically, enter anyone else; Those in a relationship can live inside the same person, for example. There is also no limit to how many layers of this exist; Someone with a hundred siblings can still hold the full hundred, with weight being the only limiting factor. However, everything inside is lightened, so it doesn't make as much of a difference."
Jeff was mostly pleased with this, and went to hit confirm... but it didn't work, instead an error message showing up. "Error. Not all required fields are set." He looked back... realizing his mistake. "OH. How could I forget the body part!"