Well, as daughter is graduating high school, knowledge and practice of proper etiquette becomes even more important. The rule here is inviting married couples as a unit. And dont use invitations or parties as ways to settle a score, it will make other guests uncomfortable.

I admit it is easy for me to say this, as my only SIL is dead.

But you want your daughter to be a gracious lady, respectful and respected. Not mired in petty conflicts.