My brother and his wife are refusing to let their kids play sports and I'm going to tell them that it is wrong.
Why do you think it's wrong for kids to not play sports?
It's important for children to develop flexibility in activities they can experience.
It's true that sports can help kids for a lot of reasons, physical and mental, but you can't tell other people how to raise their kids.
I agree, but I'm not okay with kids being taken away from sports lessons.
Rather than tell them what to do, talk about the benefits their kids will experience from team building and friendships on top of the physical aspects. Encourage your brother and sister-in-law with facts.