I think it's more like Mimi is the mom of the harem, Elma is the dad of the family, Mei is the cat herder, and the twins are the pets lol.
It's hard to really go into depth in a manga, but yeah, not only does each person have a unique personality and relationship with Hiro, but with each other...
Mei is not sitting in Hiro's lap for a reason, she weighs over 200 kilos. The dwarf twins are the second heaviest (each) followed by Mimi and Elma. They did a great job on the head patting scene. :) Thanks for the uploads!
The author has two novels/mangas going at the same time, and it's great how adult this one is drawn out vs his other one. And his other one is getting ready to have an anime dropped as well.
Sadly I don't think this one will get an anime.