My stepkids have abandonment issues with their biomom.?

I would love to know how to help my beautiful stepchildren cope with the fact that their mother abandoned them. They are 6 (girl) and 5(boy)... INCREDIBLE KIDS!!!!!! I love them more than anything, but their mother is very in and out of their lives. most of the time she calls after they are in bed so she doesnt have to speak with them, They love her. But she doesnt want anything to do with them! What do I do? am pretty much Mommy anyway, and they ask to call me mom all the time, I always say that they have a mommy that loves them very much, i dont know what else to do. I will be with these kids forever, and she wants nothing to do with them, but they still love their mom ! I dont blame them, but the hurt in their eyes- It's heartbreaking to see what this has done to them, I dont know what to do or say to make them understand that i'm not goin to leave them (they are afraid i will walk out too!) and i want to help their little hearts heal! Please help me.