Hopefully she is not on the lease, if so get out of it asap. Don’t marry this person, you and your family with never be considered before hers. Bet she will expect that all holidays will be with her family because that’s what she wants. You would not have a marriage that is a partnership, you would be the one subject in a dictatorship. You are NTJ but she definitely is!
Tell her no, she is going to hound you about making this your wedding present to her. You already did that by deeply discounting the price for making a custom dress. She’s not your friend, you don’t owe her anything.
If your brother went to court to win custody of you, you are his responsibility while a minor. By my math, SIL has been harassing you since the age of 12. You are not the jerk but she apparently is very jealous of your relationship with your brother. Until you turned 18 he was legally responsible for you, don’t know why SIL can’t comprehend that.