What I've found to be helpful is to set them up to be critical: "Yeah, so, this is an interface that my (cousin/roommate/________) designed, but I just don't think it's quite ... there. I haven't been able to articulate just what isn't right about it. I'd love to know your thoughts on it."

Set it up that way and they're primed to give critical feedback. If they can help you articulate what's problematic about the interface, they end up being the hero. You both win.