AITA for not helping my step sister with her wedding?

I was married before and are recently widowed, I was married for 5 years before we decided to divorce but my ex-husband passed away in a car accident before the divorce was finalized. Now It has been over a year and I have a new partner. My stepsister (24F) is getting married (29M) and they are having a big wedding, I didn't go to her bridal shower and I was really busy with work. I got a message from her fiancé that I didn't go to the bridal shower and that I haven't helped with the wedding at all, I am also not paying for anything nor did her parents, I was just expected to, Like my stepsister, I work in a high paying job and I make good money so I am doing well financially. She messaged me today that if I didn't pay for the flowers, cake, photographer, dj, and decorations and they want all the high-end vendors I would not be invited, so I told her na and if I am not invited I am not paying for anything, she told me to pay for everything or I will be cut off and my stepmom will cut me off too. I told her na that I will pay for my own dress, hair, and makeup as well as my date but I am not paying thousands of dollars for the wedding. My fiance told me I should just pay for everything and if I don't do it she will cut me out of her life. My stepsister and stepmom are wealthy because of their spouses but I am not wealthy because of my fiance I make good money and am financially stable on my own but they always make me feel like I am poor because my ex-husband didn't leave me a lot of money and I had to pay for his funeral and he had a lot of debt. My stepmom told me if I don't pay for the wedding she will cut me off and my stepsister will cut me off. My father told me to just pay for it but I don't want to and my stepmom told me to pay for the wedding because I am the oldest. I told her na I am not paying for the wedding and I am bringing my fiance, she told me he isn't invited and I am not invited unless I pay for the wedding, I told her I am not paying for the wedding and my stepmom and stepsister aren't speaking to me. My dad and friends are upset that I am not paying for the wedding but literally my stepsister and my stepmom who has 2 biological sons are the only family members in my life and they are only in my life because of my dad and his money. My stepsister only comes around when she needs money from my dad. AITA?