I'm a very non-confrontational person and so I frequently do not express my feelings or needs for fear of how the other person will respond. Though its something that I have been working on and I think I'm starting to get better at it.
I tend to approach things like this by trying to not make it about the other person doing something "wrong" ('cause that's just asking for an argument) but more about what I need from them.... if that makes sense. Generally the way that I know if something is a reasonable request is if its making me feel bad. If someone in my family is doing something that is upsetting or hurting me, I think I am well within reason to ask them to stop or to try to make a compromise.