Wedding Planner threatens to sue

Not well versed in Mexican law but I’d check to make sure no one signed anything. If they did, you need to review those documents.

Do you have it in writing that the wedding planner told you to take it our of the service fee? If it's in writing (email, text, etc.) and you can justify the amount that was taken from the service charge, I don't believe you would have anything to worry about, especially your aunt who has nothing to do with it.

Make sure you document exactly what the deduction was for while it's still fresh in your mind. Send that to her in an email and go from there so you have everything in writing.
I wouldn't worry about it until it become something to worry about. If she still decides to sue, you should have an opportunity to settle before it goes to court if it doesn't seem like it's worth the hassle to you.
She shouldn't be able to sue you Aunt since she had nothing to do with the wedding. But I understand not wanting to put that on your Aunt.

Hi Julieta! Congrats on your recent wedding! I am not well versed in Mexican law, however I don't think she has any ground to sue your aunt if she never signed anything. Did she say why she was going to bring your aunt into it? I would go back over the contract you have with the wedding planner and make sure no one signed anything without knowing it.