I have a question regarding how to resolve an issue with my mother. It partly relates to her relationship with my kids, but moreso to do with her relationship to me.
Are these questions considered on-topic on Parenting.SE or should I go to Interpersonal.SE?
For the most part, I think that would be considered on topic, although this might not be the best place to post such a question. Interpersonal Skills also can answer questions of that nature when they're not particularly parenting-focused, and will likely get more attention from a more heavily trafficked site.
The question Stephie linked would probably disagree, and I honestly personally would probably tend to agree with that, but it's not been the general consensus of the community as far as I can tell for years - we've permitted questions dealing with adult children, and children asking about their parents, for quite a while now.
I'd suggest the general rule: if it's a question where other parents would be specifically knowledgeable, in their capacity as parents alone, ask it here if you wish (or on Interpersonal Skills). If it's a question that another parent would not have a particular insight into, as opposed to an otherwise identical adult that did not have children, then ask it on Interpersonal Skills.
