If you think grandma might leak details, don't tell her! Just tell her the day and time. Don't even send her an invite. Have dad or someone pick her up. If she can't respect your boundaries, she doesn't get the privilege of being included.

I just have to say in response to your grandmother, and I hope it's okay to say this: What would be un-Christian is to sacrifice your wellbeing for the sake of keeping up appearances. I'm sure going NC with your mother has not been without its share of pain and loss. I'm sorry that your grandmother is confusing caring for your own spirit with being "unChristian." (I am a Christian minister. It is legit my job to address harmful theology and nip that shit in the bud. Idk if you're even a practicing person of faith and it doesn't even matter, truly, but just in case your grandma is the only source of theology in your life, I had to say it even though I know this is not a forum on religion.) I myself have family who I have had to cut out due to their harmful behavior; for me, forgiveness looks like moving forward by not dwelling on the pain they caused/letting it paralyze me, along with not continually opening up old wounds and patterns by inviting them back into my life. Please keep on taking care of yourself and cultivating a healthy community of people who love you!