2 sisters looking to ski... where should we stay?
Location-wise, the Sonne is not best-positioned for the ski-lifts and amenities. It is an uphill walk into the main part of St Moritz. However there is a bus stop right outside that takes you to all the ski areas. There is always a trade off between price and location/amenities. Do also read the reviews on booking.com, which are written by people who've actually stayed there (I haven't, and TA 'reviewers' haven't necessarily either).
The Hotel Sonne is under reconstruction at the moment, a huge building site and it will be all up-to-date soon. It's sure a very valid choice if prices are kept low as they have been before this, thus guaranteeing a good value for money ratio and I think they will keep going on with this. The location is good. It's in St. Moritz Bad, the lower part of the village and on one side has unobstructed views as there is a sports zone right adjacent. On the other side, the higher-up floors have even lake and village views. They have a good restaurant and pizzeria with decent prices on premises. The offer different rooms in different buildings and thus at different prices.
This is a walk of ca. 7 to 10 minutes to the Signal Cablecar valley station from where you access the Corviglia skiing area or you can take the bus directly in front of the hotel to get up to Corviglia or even all the other areas like Corvatsch and Diavolezza. So really a good location for skiers and on the level part of the village, so no uphills from there and a lively part as this is where most of the locals live.