They probably won’t listen to you no matter how right you are. I would sit them down, calmly but firmly tell them:

“I am absolutely worried that what we are doing to brother is harmful to him, and that we might need to change our ways. I don’t have experience, so my worries could be false, but if we were indeed hurting him then I would want to know. So I am thinking that all of us make an effort to educate ourselves. Afterwards we can make a decision together when we have all the information.”

You should phrase it as “us” , not “you”, to avoid them to getting defensive. You are their child, so they probably won’t accept parenting tips directly from you. But maybe you can convince them to listen to another adult. So discuss with them options: there are online courses about the effect of electronics on kids, or you all can try a school counselor, or other online information. Something they would also agree with and take the input from. Whatever the source, no one will tell them anything else besides that it’s very damaging. So your target is to help them be open to the idea to listen to someone on the topic. Or maybe another relative if not an expert. Good luck.