this is just a distraction that makes those not checking their phones become fed up with the lack of concentration and attention and just decide to pick up their own phone as well.
This is very true, although not in a family gathering scenario now but in my own conversation with someone. Mere conversation with someone and I find out the person is distracted by the device in his hands, I automatically feel my words are trash and not making any meaning which is why the person feels the only way to lighten up the atmosphere is to get busy with phone. Such act can lead to the end of the conversation no matter how such person claim he was listening.
Your sibling missing such an amazing appointment shows that complete ban of phone for adults can take a negative turn. Reversing the rule was the wisest things to do as far as everyone understands the instructions.