I agree with your husband-it is not her birthday so she shouldn't be receiving any gifts.
Instead of threatening your daughter to behave that morning, I suggest you get her involved in the the party now. Tell her you need her help and ask her to be your "Little Party Planner". And make a big deal about it. I know the party is already this Sunday, but if you can, go somewhere only with her to "plan" the day. Give her a clipboard and make a list together. Get her excited about your son's birthday. Maybe she can help you bake the cupcakes, assemble goody bags or decorate. The morning of the party, give her some sidewalk chalk and have her decorate your front walk. Have her write "Party this way" with arrows and Welcome to ______'s 6th Birthday Party!" It will keep her busy and out of your way. During the party, give her specific jobs. (Of course, you can't just ask her to do these things, you need to "put her in charge" of doing them.) Have her take the guest's coats when they arrive, pass out the cupcakes, and/or write down who the gifts are from when your son opens them. She is even old enough to help run a game. Keep her involved and she will be so excited she won't even notice it's not her birthday.
Also, let her help you surprise your son. Have her make a card and pick out her own gift for him. And if she has any money of her own, have her contribute to his gift even if it's just $1.
As for your 2 yr old...distraction, distraction, distraction. Give him a bag and have him collect all the wrapping paper when the gifts are opened.
Good luck and have fun!