My ds13 has an older half sister (14). She lives a distance away and hasn't been to their dads in a few months, or very regularly over the last couple of years, but will be over next week.
My DS and her used to be quite close, despite the distance. But tonight I was asking him if he was looking forward to seeing her and he basically said not really, he wasn't bothered, and he doesn't miss her. I felt a bit sad for them.
He seemed quite angry a while back that she wasn't coming to see them, and I don't know if there's still a bit of resentment there, or if he really just doesn't care.
Is that okay? 
Is it an age thing? Is it just how it is sometimes? I don't want him to be actively sad over her, but I would like them to have the bond that they used to.
Thanks.