My sister is getting married in church in four months, and I can't accept it.

I'm older than her, but I'm autistic, I don't work, and I live with my parents, so I'm forced to attend. She and I see each other regularly, but we haven't had a real relationship for years. I'm not the least bit happy for her. Plus, I'm an atheist, and it goes against my principles to attend a religious wedding. I'm thinking of therapy to come to terms with this. What do you recommend?