Translation:Someone was looking out the window of my sister's bedroom.
Depends where. Does she have multiple rooms? Is only one of them the bedroom? I wouldn't normally call a kid's room ložnice btw, unless it is dedicated for sleeping and they have some additional rooms. For me, ložnice is a place you go to sleep, not where you work, play during the day etc.
Thank you so much for your responses. I'm still trying to understand. In English I would refer to a child's bedroom as a bedroom or a room even if it has toys, etc. I don't think I'd refer to a child's room in any other sense than a bedroom. I guess it's a little different in Czech