Based on my experience with my kids and their various grandparents - blood and step - I am sure you are building a wonderful relationship with your grandkids. Good for you!
When my daughter got married, she invited all her grandparents, of course, but she also invited one of my Dad's former wives - which I guess makes her a "former step grand."
I thought it wonderful that she still has such a good relationship with this wonderful woman... and that all the grands (pretty much) behaved themselves at the event!
To date I don't have any grandkids, blood or step, but I am hoping to be a good grand when the times comes.