I don't think there's enough info given here for us to be able to help much.
For ex. if this is a close family member who you don't see very often, repeatedly turning down their offers to meet up might effectively be severing a family tie. Is that what you want?
Income levels change and bored teenagers grow up. So it's also possible that you'll enjoy these dinners more sometime, maybe sometime soon.
You might like it better with the other people there. Perhaps they find these meet ups unsatisfying too and are trying to improve them by including more people.
And how close of relatives are they?
Also, can you meet up with them in a different way that you might like better, like having them over to your house or whatever?
And, I don't get why you care if the teens are on their phones, unless you're also a teen and feel snubbed by them or something. It sounds like they're quiet and inobtrusive and just don't really want to be there, either.
You also didn't say if the check is split, if you're paying for part of the disinterested teenagers' desserts etc. Or if you just don't think they deserve them or what is going on with that.
Are you bringing anything to the table to try to improve this situation or only silent complaints? Is there anything you can do to improve the meet ups with them?
So, if you explain the situation more, you'll probably get better advice.