WIBTA If I tell my SIL I prefer to look after my own kids, not hers?

As renowned researcher Dr. Brené Brown explains, “Boundaries are the distance at which I can love you and me simultaneously.” This situation highlights a common challenge in family dynamics where the expectation of mutual support clashes with the need for individual responsibility and defined personal space.

The OP and his wife operate under a clear model of parental ownership: they are fully responsible for their children at all times. The SIL and her husband, however, appear to utilize social settings as opportunities to offload routine parental duties onto available family members. While the tasks requested (retrieving a cup, attending to a diaper) are minor, the *pattern* of asking—especially when both parents are demonstrably idle—signals a lack of respect for the OP's established boundaries and an assumption of available service labor. This can lead to resentment, which the OP is currently experiencing, feeling 'petty' for wanting to enforce his own standards of care onto others' children.

The OP's desire to state, "I don't ask you to change our kids' diapers or grab their sippy cups from the next room, don't ask me," is fundamentally appropriate from a boundary perspective. A constructive recommendation would be for the OP to use 'I' statements focused on his own responsibilities rather than criticizing their parenting. For instance, instead of refusing a request directly, he could respond by saying, "I need to focus on my youngest right now, so I can't help with that," or, "My wife and I handle our own kids' needs, so you'll need to take care of that one."