You can't make people interested, but the way I see it you've got two choices: either accept that's the way they are and that you have to put all the effort in for little return, or let the relationship take it's natural course (ie if you stop bothering, they won't either and it will naturally diminish).
I used to make the effort with the inlaws, make DH phone, go over etc etc, but they were never interested, never kept up with what the children were doing, never asked about them and their interests etc... Then DH told them how much it hurt him. Dd had chicken pox at the time, and they didn't even bother to phone up and ask how she was feeling. So we stopped making all the effort. I honestly don't think they noticed for the first year or two and when they did, well it was too little too late, I guess. Now, we see them once or twice a year, when they want to. But the girls have such busy lives, it's difficult to fit everything in.
They came over at Christmas, and didn't ask the girls once whether they had a good Christmas, or what they were given. The girls now have lost interest. When they did come over, they said a polite hello and then played on their iPods. I did think about stopping them, but then thought sod it, why should I bother when they can't even manage the niceties?
You can't force them to be interested. When your DC are little, you might find it manageable to do all the running around after them, but as your DC grow, and you're increasingly busy running a home, looking after your DC, working, doing all the hobbies etc... Your time becomes increasingly precious, and this becomes harder to sustain (ime). My life is incredibly hectic these days, and if I get an hour or two to myself, i consider that a good day. I found it unsustainable, but don't expect them to pick up the pieces. If you don't put the effort in, and they're not interested, it's unlikely they will either.