Not going to my sister’s wedding bc she won’t let me bring a plus one

My sister on the my biological dad’s side is getting married and she did not account for me bringing my significant other, whom I’ve been with for five years. My sister and I aren’t that close btw. I already didn’t want to go to the wedding because her whole family (and technically my family by blood) would be there and I have never met them. It was an abusive relationship between my biological dad and mother and so I haven’t seen that side of the family since I was like 5. I assumed I could bring my significant other but when I RSVPed, I didn’t get an option to put him down. I messaged my sister and she said that at the moment he couldn’t come but would let me know later on because there was limited availability. I told her that I loved her and would love to celebrate her but if my significant other couldn’t come I’d have to miss the wedding, as I would be uncomfortable being around an entire family that I’ve never met but knows me while being alone. As an anxious person who has never wanted anything to do with that side of my family, I couldn’t fathom being at a 6 hour wedding by myself. Am I the asshole?