We went for one week with my wife and our 2 children at Xenia's beach house, we had an amazing stay. Vathi is a very pleasant and calm bay, with several small restaurants on the beach, and a little supermaket you can access within walking distance.
The place is very quiet and unspoiled, you do not hear much from the garden except the sound of the bells of wild goats living in the mountains, and of the waves.
We had dinner every evening in the garden, facing the sea, watching the sunset.
Kostas' parents were very friendly, with little attentions for us, and were always there to recommend us the nicest places to be seen in Sifnos. Our stay was really really pleasant and relaxing for the whole family.