The cousin you only see at holidays is part of a short list of people who knew you at six and at sixty. Keep them.

Very few people get to see both ends of your life. Somebody who met you as a child and is still around decades later holds a continuous record nobody else has, however little you actually talk. That continuity is rare enough to be worth some inconvenience.

Get a number for the cousin you only see once a year and use it once outside the holidays. Low effort, kept up over decades, is the whole method.