AIO for telling my SIL she can’t “claim” godmother just because she said it first?

I’m due in a few weeks. My SIL (husband’s sister) is… a lot. She’s always been the center of attention and struggles when she’s not.

Last month, she announced, publicly on social media, that she’s going to be the baby’s godmother. We had not asked anyone yet. She tagged me and my husband, and captioned it, “So honored to guide this little soul 💕 #GodmotherVibes.”

I was shocked and privately told her, “We haven’t picked anyone yet, and that’s not how this works.” She got quiet, then told me she “just assumed” and “called dibs.”

Well, we’ve decided on someone else, my best friend of 20 years, who’s been like a sister to me. When SIL found out, she absolutely lost it and accused me of “stealing her role.”

Now my in-laws are telling me I should’ve just given it to her “for peace.” But is that how this works now? First come, first served??

AIO for telling her she doesn’t get to just claim a title?