AITAH because I refused to speak Spanish with a customer after being told it wasn’t welcome?

I am a part owner of a store that sells suits and rents tuxedos. Obviously we deal with wedding parties. I was born in Canada but I am a native Spanish speaker since that is all my family spoke at home. I used to also traveled to my parents home country every summer to stay with my grandparents and visit with the rest of my family. I still go to visit but just for a week or two.
We had a large wedding party come in last weekend. They were renting six tuxedos and buying two suits. The suits were for the father of the bride and her grandfather. I called over one of the assistants to help me with a large group. Our tuxedos are all made to fit a wide variety of bodies and be temporarily altered for a good look. Tammy my assistant is awesome and can mark up the alterations no problem. Our suits are for purchase only and since they can be expensive I usually take those myself.
I noticed that the family was speaking Spanish and I make it a point to speak Spanish with clients at the beginning to keep them from trying to be private in their conversations and then embarrass them when they figure it out. Long experience has taught me that people don’t like that. I greeted them all in English and then again in Spanish. I continued in Spanish to ask them what kind of suits they were looking for.
A young woman in the group literally sneered at me and said I didn’t have to speak Spanish to them because they all spoke English. I’ve never encountered this attitude before so I apologized and continued in English only.
All was going well until I got to Grandpa. He absolutely did not speak English. I went to the young woman and asked for her help translating. She was watching her groom getting fitted so she wasn’t busy. She came over and translated everything her grandfather said to me and then back to him. By the end the old guy looked great in his new suit and she had completely missed the other fittings.
She got mad at me for making her miss out. I reminded her that she told me to only use English.
A few days later she posted a review saying that I was unprofessional and made her translate when I spoke fluent Spanish. I responded with my side and offered to provide a video of her telling me to speak English. She removed the review when people started mocking her.
She called the store and threatened to cancel the order. I agreed that might be best and said she could bring her receipt in for a full refund. The suits hadn’t been altered yet so some steam would get rid of the chalk marks no problem. I would call the supplier and cancel the tux order as well. I even offered to refund the deposit on the tuxes.
Her mother brought her in. She apologized for the way she spoke to me and said that she would like to keep the order the way it was. I accepted the apology and told her that I understood that weddings are stressful and such.
My brother, who is my partner, said I was a dick for not just talking to the old man in Spanish.