I really feel uncomfortable that my in laws let themselves into our home with a key (which is supposed to be for emergencies!) when visiting us but don't know if IABU?
It's honestly not personal to them as we get on fine, I just don't like that they just come in without knocking or anything - sometimes it's given me a fright when I don't hear the door unlocking and other times they've almost caught me in the bathroom 
I don't want them to feel unwelcome and get the feeling it's the norm where they live (amongst family members anyway) as it's more rural whereas we live in a city.
They were visiting over the weekend and did it again and I tried to make a joke of it "oh you frightened me, haha" but I think I need to be very clear and say politely but firmly that I'd rather they rang the bell so we can let them in - that isn't too rude is it?