AITAH for not attending my sisters farewell party?

My half sister is moving overseas for 3 years and having a farewell party of about 50 people. She invited her father (he was apart of my life for 20 years but not biological) and i dont want to go because i cut him off about 2 years ago because he is a narciscisst and treated me like crap when i was younger. The last contact i had with him was a negative message he sent me on my wedding day for not inviting him (which my sister kicked the fuck off about because i didnt invite him and she thinks he shouldve been there). I feel stuck because I dont want to let my sister down but my mental health is already suffering because im growing in anxiety about seeing him. AITAH if I dont go to the party?