Unless you are renting a grand villa, you most likely are looking at a two bedroom villa...so that would be a studio villa attached to a one-bedroom unit. A queen bed and a double sofabed in the studio, a double sofabed in the living room and a king bed in the master. One adult couple in the living room (hope they can take the sofabed...it can be uncomfortable), one adult couple in the master, and the family of 5 in the studio. A two-bedroom villa, regular season is about $725....so, I would try to figure out a way to break it into 3 pieces, with the family of 5 paying more. Maybe divide it into 3, and then sister pays $242 per night, whoever gets the sofabed pays $200 per night and the master pays $285...or something like that. I really think whoever gets the privacy of the studio section should pay more.
Now, if in fact you are renting a grand villa, then I would divide it based on beds used or on bedrooms. Maybe you should think about getting a one bedroom and a different studio entirely. Or a twobedroom and a studio...you keep the studio and let the rest of the family duke it out!!!