What are the laws of tznius in your home if a neighbor can see through your windows (for example in the kitchen or hallways)?

In general, if people can easily see you e.g. through a window which is directly opposite a neighbor's window, or a large living-room window, and especially at night where it's dark outside and the room is well-lit, then it's the same as in the street, so you should dress properly, e.g. hair covering for married women and not wearing shorts. On the other hand, if you're not exercising by the window, and you just want to go up the stairs, you can wear long pants or a shirt where the sleeves are a little short, for even if they see you, that short time doesn't enable them to focus on you. Most people just buy blinds or curtains, which easily solves such cases, if you wish to walk around more comfortably or freely. On the other hand, if an outsider must make a significant and conscious effort to "peak" in, say during the day where it's difficult to see from the outside, or if a neighbor's windows do not enable him to see you inside without him making a significant effort, then that's his problem, and you need not inconvenience yourself.