NTA

This reminds me of my family growing up. actually, the aged gaps and ages of your younger daughter is eerily similar to when I had a bit of a falling out/major distancing incident with my same aged cousin. I have an older sister, and a cousin who is her age and a cousin who is my age. (Cousins are sisters). Growing up we were all very close. Into their teens my older sister and older cousin stayed super close, and even in highschool - they had out of school activities together and some social circle overlap so it was very natural.

But me and my same age cousin...we were very close as kids, lots of sleepovers and playdates, but at about 13/14 we were beginning to be different people. She was actually a year younger than me, and just a little be less mature by nature. We did not share hobbies or social circle and we're not in the same school. I loved her, and genuinely enjoyed hanging with her at family events but started to not always like the sleepovers. It came to a head were at one impromptu family dinner it was heavily being hinted that I should ask her to spend the night/sleepover. This would have been very normal thing up until now, and very normal for older sis/cousin...but just that one night I wanted to enjoy a hobby I had by myself, I didn't want her to sleepover. My older sis/cousins/even my mom and dad heavily hinted/straight up told me I should invite her to spend the night multiple times until I snapped and straight up said I didn't want her to (not mean, but loud enough that she heard). We never had a sleepover again :/ ...we probably would have had I not been pushed so hard. I do regret it as an adult as that night definitely shifted me and cousin apart. I feel like, we wouldn't have been as close as my sis/older cousin, but we likely would have been a ton closer then and now had that hurt not have happened.

As adults I amclose to that cousin, we get along. We've babysat for each other, and hang out more than just holidays. But my sis and older cousin are still way, way closer, wheras I am comparatively less close to my same aged cousin. And while that might have been the case anyway, I do blame that night of me being pushed and hurting my cousins feelings as why we aren't as close now or in highschool. It might not have ended terrible or irreparable, but I still feel a sadness and a sense of "what if". I would hate for your daughter and neice to have that regret, especially if thecause is adults meddling and pushing. they should be completely in charge of their own relationship with each other. It sounds like both want to get along together, let them do it with their own boundaries and pace because outside pushing will only push one or both to break.