How do you deal with people who never let you finish a sentence?

I don't even mean people who will butt in to add something to the conversation. I mean people who will ask you a question, cut you off while you're mid sentence and then just change the subject entirely.

I have this problem with some people on my husband's side of the family. I find it exhausting and so annoying. I can never say what I want to or respond properly to what they ask me. It's never a conversation. I'm just being talked at.