I was in a very similar position to you once.
I met my ex when we were at college, he left after the first year to go to Australia for a year (already arranged and planned before we met). It broke my heart when he left. I pined for him for the year he was away, and when he came back we did meet up to go to a mutual friend's birthday party. I honestly thought we were going to make another go of it, but I lived 60 miles away by that point. He started seeing someone else, who he eventually married and had kids with.
I lived in hope that we would work things out, but it just wasn't to be. I thought about him every day for about 4 years, missing him and loving him in his absence.
Eventually I moved about 200 miles away, met Mr P and 12 years and two kids later I couldn't be happier.
I think it's entirely up to you if you do get in touch via his mum or sister, but if you do be prepared to have your heart broken all over again. He may have moved on and be married with kids himself by now.
Whatever you do, good luck.