As my girls have gotten older, I've turned over the responsibility of having a relationship with their Dad over to them to do as they choose. However, call me crazy, but I still send photos, email updates, etc. to their Dad. He and I share these kids, regardless of where and how we live nowadays. I do it because it makes me feel good to keep him in the loop, even if he doesn't acknowledge my updates (sometimes he doesn't...I just assume he's busy) or thanks me for the photos. I do it for me. This is the person I want to be.
Where the boundary is: not getting in the middle of the drama or whatever is going on at the moment between the girls and him. Most of the time, that's easy to do, but sometimes I mess up and the girls get mad at me, or my ex does. Oh well. I try!
My suggestion would be to find your comfortable place in this. You're their Mother; he's their Father. You both are invested in these kids, no matter how old they are. Decide what kind of person you want to be here, and be that person, regardless of the outcome. Do what you feel is the right thing to do for you.