If someone is changing a baby's diaper in the dining room, kitchen or other places food is consumed, can't you just speak up? Has no one heard of the phrase "don't shit where you eat"? Shit's shit, there's a reason why it's coming out the ass smelling the way it does, and it ain't because it's a tasty fudgy delight.
Here's how I escape forced responsibility over children. Play the party animal for all it's worth. Family drinking wine at dinner? Make sure you either, a. drink twice as much as anyone else, or b. manage, somehow, to get smashed on whatever you're alloted. Discussions of personal life? Mention bars and alcohol as much as possible, or develop a very child-unfriendly passion - firearms, axe throwing, underground fight club, vintage porn collecting, which you then proudly share with all your childed relatives, "Here's a picture of me dressed as a zombie for Easter", "And this is the person I'm writing 'adult' novels with", "Careful with that axe, Sproggliee". When they bring out Sprogette's ass and assorted waste for all to coo over, invite the family in to see what you made in the toilet, "It looks like a fudgy Nixon!" Decorate your home in a very child unfriendly manner, lots of sharp objects protruding at knee height, glass shelves, exposed wires snapping with electricty, assorted and colorful pills laying out on the floor, porn, booze bottles and lots of them. Don't forget to be extra nice to your nieces, nephews and little cousins - make sure they know that the only way you can be super nice to them is because you have no children of your own. Be the good example of the fun of childfree-ness, fill 'em with candy, make sure they know that having children is a choice, in spite of what the rest of society may say, and smile as you give 'em back to their parents.
Seriously, you probably already live a life that is very child unfriendly. If not, make one up. If your relations don't know enough to not bingo you, then they probably aren't ever going to figure out that your life of base jumping, drug use and drunken binges isn't real. I make sure those I suspect may bingo me know I go to bars, rock shows, do lots of volunteer work, drink, play violent video games and engage in a very physical sport that I could never do pregnant. They know know I am very happy, and I make sure they have the impression that it would be irresponsible, if not criminal, of me to have children of my own or be put in charge of the children of others. Yeah, I'm actually a responsible and upstanding member of society who spends a majority of my evenings quietly at home, but do they have to know about that part of my life so they can add fuel to their baby rabies fire? No, I don't think so.