We have rented our villas from VRBO.COM. They are privately owned. Many owners are from the UK.
Personally, we have rented with family members twice before and never again.
For instance, some people like to leave the villa fairly tidy every time they leave it, others could care less and leave clothes, dishes, half eaten food and wet swim suits all over the house.
Some like to go to bed at a certain hour, others may like to stay up drinking half the night.
Some people's children whine constantly, others, not.
Some people let their children take dvd's out of the villa for the 10 minute drive to the park EVEN though there are signs that specifically state not to remove dvd's from the villa. If you object to this, the other parent (whose name is not on the lease and did not pay the deposit) may tell the other parent to RELAX because the kids will put them back afterward. Then, a week later, when you are dropping off the rental car at the airport, you find a pile of said dvd's under the seat.
Or so I've heard.
My advice would be if you are going to do it to have at least 3 bathrooms with 3 families.