Postponed Probs

My fiance and I decided a week before the wedding to postpone. We figured the universe was trying to send us a sign because his work put a travel ban in place, his sister injured her leg and she would have to use crutches at the wedding, and my maid of honor has a scratched cornea and would've had to wear glasses that she hates so we decided with all of that to postpone until next July. Our venue and other vendors were extremely understanding when I explained the situation. Our venue was a hotel so they gave discounts to all of our guests that were planning on staying there for the wedding. The problem I'm having is that a month ago my sister started harassing us about her step-son. In December she eloped without telling anyone until after it was done. She gained a six-year-old step-son from the marriage. My fiance and I already selected our flower girls (his two nieces) and our ring bearer (his nephew). My sister has been hinting that we should make her now step-son a ring bearer since we have two flower girls she thinks it wouldn't be any big deal if we made her step-son a ring bearer to even it out. Otherwise, the step-son won't be attending the wedding because we aren't inviting children with the exception of the flower girls and ring bearer. I barely know her step-son as my sister has only been with her husband for like ten months. I don't really feel comfortable with him being in the wedding whereas I'm close with my fiance's nieces and nephews and always envisioned them in the wedding. Also, we would have to pay for the outfit her step-son would wear because my sister and her husband struggle with money and I don't want him in the wedding to begin with so I definitely don't want to pay for his outfit. I feel like she is going to continue to pressure us for the next twelve months until she gets what she wants. Are we wrong for not wanting her step-son in the wedding?