You are writing about your daughter's mother. That is probably why the woman has been included in so many situations. There may be some undercurrent there too--did your mother disapprove of the divorce? Or of your choice for a second wife? Is your ex-wife still single and alone? Or it could be that your mother simply enjoys exercising her own power to choose the people she likes at the events she holds.

I'd accept my daughter's decision gracefully and then invite her to go out to a celebratory lunch or dinner at some point after the graduation, just you and your daughter and step-mom, if she gets along with daughter.

The best advice I can give to a parent is give what you want to give and are able to give, love unconditionally and don't count grievances.