Hello. I cannot help you with a flat rental, but have you considered booking a guest flat for your sister and Mum to stay in during their visit? I know they only provide a bed, and bathroom, and a kettle, but it might be an alternative choice for you.
Thank you very much for the tip. I'll check if a flat is available.
The Estate Office deal with all bookings. There are 3 double rooms, and 3 single ones, based in Crescent House. Hope you manage to get something sorted.