AITA for declining being a God mother?

My (29F) sister (32F) is having a baby this May and so far everything is healthy and going well. My sister has sadly had 4 miscarriages and this is their rainbow baby (I think that’s what they’re called). I love my sister dearly and she’s been trying for a baby for years. My sister and I are best friends and have been ever since we were small. I’ve helped with the baby shower, crib building, (her husband was busy with the military and couldn’t help at the time) making the nursery, and basically all the things. They’re currently keeping the gender a surprise until delivery and will also decide a name then as well.

Well, my fiancé (30F) and I have both decided that we want to be child free as we both like children but we both don’t want any. We do have 2 cats named Squish and Boulder and they’re the best.

Anyways, my sister and her husband took my wife and I out to dinner and asked if we could be the godparents to their child. My fiancé and I were honestly stunned but after a very long and awkward pause I said no, and then hell broke loose. My sister didn’t believe me at first and then started crying and kept asking me “why” to which I told her that my fiancé and I don’t want kids and in the event that something did happen to the both of them we wouldn’t know what to do. I told her that she knows we chose to be child free but she thought I would change my mind if it was my niece or nephew.

I told her that she should choose people who want kids/already have them and she got mad at me and grabbed something from her purse and threw it at me. It was matching godmother shirts for my fiancé and I, and then they both started creating a commotion. Her husband called us selfish and my sister would not stop crying while saying I should know how important this was to her. Well the restaurant eventually asked us to leave, to which we did and my sister has been ignoring my phone calls and texts and I feel terrible. My parents are staying out of it but my brother is saying I should have just said yes. AITA?

Edit: when I say “godparents” I mean that they will designate us as the people who take guardianship over their child if they both die. We would legally be their child’s guardians.

Edit 2: Cat tax is my second post on my profile, the one on the left is Squish and the one on the right is Boulder.