He comes to a decision, and selects "Oldest sister", deciding the oldest would probably be better. Then, he gets to work on the specifications.
"By default, everyone on earth will live inside of their oldest sister; The oldest sister in the family will house all of her siblings within, and while feeling the weight, it'll be halved."
"If the oldest sister is not the oldest sibling, the older siblings would still live inside the girl; It's simply the first living one. If any incident happened to the oldest sister that resulted in death, then the next sister in line will hold all of her siblings instead."
"When one does not have any sisters at all, or is the oldest sister alive at that moment, they will live where their parents do; Whether this is in a house, or inside of someone else. They may alternatively, with permission, live inside of a close, female friend, regardless of age. It is possible to live inside of any female, but it's only the oldest who usually has others living in her."
"Moving out is still an option, and when the oldest sister does so, all siblings will still remain inside of her and move with her. When inside, there would generally be no issue with seeing a sibling naked or anything; Cousins would be slightly weirded out, while anyone not related will view it the same way as it is currently."
"Having sex inside another person is generally uncommon, but not entirely prohibited. This would be seen about the same way as normal, without any real modifications."
He decided this was good for now, but wanted to think on another possible change... On one hand, it'd be a nice quality of life change, but on the other, it could be more fun without it...