Well, your issue isn't so much a "step parenting" issue, as it is a parenting issue.
I know a family where the mother is Orthodox, and dad is RC. While he's non-practicing in his own Faith, he takes every opportunity to ridicule Orthodoxy and let the kids know that it's a waste of time and stupid.
They are married, living in the same house, and mom is trying to counteract dad's negativity and poor influence on their children.
Just reiterate the Faith in your home, when the girls are with you. God isn't only found in church. Start slowly at home. Stand in your prayer corner, with lit candles and pray together. Do charity work together, teaching the girls that Christ instructed us to care for others. Read children's books about the lives or saints, or stories from a Children's Bible.
Just do what you can with them at home.
However, do tell them about church, about Orthodoxy, about Christ. Go on the Web and show them photos, show them photos of kids standing in line for Holy Communion, having fun in Church School....try to create an interest, so the kids develope a desire to go to church on their own.
Have the children join you in praying for their mother, that God should soften her heart, and that she should know the joys that come with knowing Christ.
Don't say anything negative about their mom (that might backfire as kids are often protective of their parents), but, state that mom would be happier if she knew Christ.....one day we'll ask your mom to join us.....your mom would have such fun getting an Easter basket blessed.....be positive and upbeat.
Will pray for your situation and all the other families (and there are many of them) that find themselves in similar circumstances!