My parents used to make very blunt statements about how other people, especially family, were failing in certain areas. My father's favourite thing to say when he saw me was "you need to wash your hair, it looks greasy." Often only a few hours after I had washed my hair, too.
I just put up with it, because I had grown up that way, until DH came along. While we were just going out, I didn't want him to have to have these things said to him. It's easier to stick up for someone else, I guess, because they often matter more to you. So when they said something rude, we would just say calmly, "wow, what a rude thing to say," and then carry on the conversation among ourselves. They would sometimes stew in silence, but we would be so busy talking that it was easy to pretend not to notice it. Usually they would protest, though. Looking back, we should have just gotten up and left the room, even mid-meal. I recommend you do this: "that's a very rude thing to say," before leaving, if it gets too bad.
With my parents, it didn't make them change their ways at all, but it felt good.
And both my parents used to do this really annoying thing with entering your room: they would knock as they walked in. The knock was completely superfluous. I can't tell you how many times, as a body-conscious and awkward teen, they walked in on me changing like it was nothing. To this day, a knock at the door makes my heart pound. I learned to have a messy room all the time, so a pile of clothes or books would buffer the door. They would start to open it, but it would hit a noisy pile of something and they would pause a bit, startled by the sound, before continuing to enter. It gave me a bit more warning. As I got older, I would often have chairs or other furniture in front of the door ("I was just moving it to reach something"), so they couldn't enter at all. Maybe you can leave your big heavy suitcase in front of the door and keep forgetting to move it. "Wow, knock next time, so I can move my suitcase out of the way!"
It sounds like you're on the right track here: a distant relationship is probably the best you can hope for. It'll be hard, but you can practice on your next visit. Don't get engaged in conversations beyond "uh-huh," "that's nice," and "oh". Bring lots of books that urgently need reading.
And I'd like to second what has already been said upthread: consider carefully if you want your possible future children around these bullies, even supervised. Unless they change dramatically, your children will grow up seeing their rude behaviour, even if you deflect it or outright leave every time. When I realised that any children I might have would never meet my parents under any circumstances, it was, frankly, a load of my chest.