Lets start by saying that indeed it is a beautiful (and cosy) house! With a really nice (and large) front-porch. We were welcomed by the owner in a very personal and friendly manner. They had put some necessities in the fridge. The house is in a real quiet part of Apollonia with a beautiful sight of the city. We felt at home right away.
The house is some 200 years old, but recently 'refurbished', and one of the nice aspects is that the house is really cool at night, just with normal ventilation (we didn't have to make use of the airco).
We, a family of four, can really recommend renting this house and enjoying all Sifnos has to offer. And that's a lot!