First, I'm sorry this is so hard on you and on your daughter. It's never fun to feel left out. Because of that, before I say anything else, my advice to YOU as her mother is to help her be OKAY with the situation as it is.
"_________ is SS's grandmother. Sometimes we have special relationships with a person in our family and like to do things with just that person. And that's great. I have a special relationship with you because you're my daughter. Why don't just you and I go have a girls day!"
Now... You've only been married a year. You cannot force family blending on anyone, much less everyone involved. It's not reasonable, or healthy. If you want this woman to have a relationship with your daughter, let them develop one. Just because you married her son, doesn't make her your daughter's grandmother. She shouldn't be made to feel bad for continuing the relationship she already has with her grandson because she doesn't have the same one with your daughter. And your daughter shouldn't be encouraged to feel bad for not having the same relationship with her step-dad's mother that her step-brother has with his grandmother!
Also, keep in mind, she HAD a close relationship with a step "grand child" and that marriage ended. I don't know the ongoing terms of that relationship, but it's something to consider before framing everything in terms of your own perspective.
FYI, it takes the same number of years as age of child at the time of your marriage for a family to be totally "blended." A year in, the kids don't really feel like siblings and the new "grandparents" are just nice old people, and it has nothing to do with not being taken out alone. Give it time. Let it happen. FWIW, It's been four years and my son is now totally comfortable and easy with his "step" extended family. He's over there all the time, and invites them to school stuff.