I'm going to my grandmother's birthday party even if it will hurt my mother.
Why would your mother be hurt by that?
My mother has strong feelings about my grandmother, so it will hurt her if I choose to go.
While family relationships can be difficult to manage, you have to decide if you will hurt your mother by going to the party, or hurt your grandmother by not going. You have to choose the option that is right for you and that you can live with.
That's a good idea, thank you.
You're welcome. Sometimes you're forced to make a hard decision between family.