Some individuals know early on that they don't want to have children. They make the choice that they consider is best for them. If one thinks that they would enjoy life without having to look after some tiny tot monsters, that's their prerogative. Some folks would even go so far as to explain to their in-laws that they don't want to be on babysitting duty because of their preferences, which is also fine—finding childcare outside of a family circle should be something to consider when someone has children. It shouldn't be your family's problem.
In this Reddit story, a child-free woman goes on a family vacation with her immediate family and in-laws. Her sister-in-law has six children, two of which are fathered by the original poster's (OP's) brother. The OP expresses that her brother and SIL's arrangement is that she stays at home with the children while he supports the family financially. Unfortunately, this arrangement ends up biting the OP in the back. During the family vacation, OP's sister-in-law decides to repeatedly ask her if she can watch her children so she can relax. The OP, though, has always been transparent about her stance on being child-free and everything that entails. Instead of getting angry with her husband, the SIL places the blame on the OP. Seems a bit backward.