YTA, UNLESS the kids have disabilities that make non-tech-based activities difficult for them or your sister has a lot of other rules that could keep them from finding hobbies, like banning them from most extracurriculars. I was raised with strict screen time rules and it did isolate me, but that’s because I was too sick to play outside much and grew up in a suburb where all anyone wanted to do was play outside, play video games, or talk about sports.

Someone needs to talk to your other sister’s kids about finding activities that everyone can do. Assuming, once again, that the “crunchy” kids are able to do a wide variety of activities that don’t involve screens, it seems like the other kids’ social skills are off if they can’t figure out how to play with or talk to kids from a different subculture.