I stayed in this apartment for 1 week and it was perfect! Sandy is very friendly and communicative and picked me up at the port when I arrived (if it's your first time to Sifnos, note that cars aren't allowed into Piraeus to pick up ferry passengers, so you'll need to walk about ~5 minutes down the main road to the parking lot -- I was a bit confused when I arrived but we managed to find each other). The apartment was just as pictured, small but modern with fast internet and lots of helpful extras (water and juice in the fridge, clothespins for the drying rack, etc). While the bed is a sofa bed, there is an additional thick mattress on top of it, so it was quite comfortable. I got to meet Sandy's mother and her friend when I checked out, and they were warm and kind, and helped me find my way to my next Airbnb on Sifnos.
If you're debating where to stay on the island: I think Kastro is the most beautiful and peaceful town, with two restaurants I loved (Astro and Leonidas), gorgeous sunrises, and easy access to/views of the sea, but if you don't have a car and prefer not to take taxis, it's definitely less convenient than Apollonia/Artemonas, which is where the buses and many of the trails originate and which have way more people and dining/shopping opportunities...I split my 2 weeks in Sifnos between Kastro and Artemonas, which felt like a good balance.