How to deal with someone who constantly interrupts?

I have a family member who has this INCREDIBLY annoying habit of constantly interrupting when someone else is speaking. I have no idea of how they cannot see how rude this is!!! When I'm in the middle of saying something and they do it to me, I say 'yep well what I'm saying doesn't matter anyway' but they don't take the hint. Any advice before I go clinically insane?