Your husband needs to treat you like you are the bell of the ball, and his attention should be focused on SD, new SNL and you. That will help calm most notions to bash you. Weddings and funerals are places where hatchets are buried, even if only for a short time. Alcohol may not be your friend, but if tongues get loose keep your composure. People see very quickly who is the integator and troublemaker. 

I wish you the best. I get along my DH 1st ex (not friends, but we are considerate at events to the point of occasionally sititng together). I have already made it clear that when the youngest SS gets married I will not attend if BM will be there. It's not worth my stress and headache. 

And.... get out of your own head.

Their wedding is after  yours. Focus on yours.  For F's sake inform your FDH that not one penny of marital resources will go to pay his adult daughter for anything other than what has already been provided for their wedding. Period. Dot.

If they want to take her car to Hawaii, they should refinance it with a company that will allow for it to ship to Hawaii.  Their problem, not your's or  your FDH's.

Go to their wedding with your giant rock of a ring on high bling, dressed to the 9s, in your new favorite boutique fashion ensemble and rocking your new make over confidently beaming your happiness on your DH's arm.  His Xs can suck a rock if they have any issues over it all.  Not..... your.... problem.

Script your response to any toxic crap they may pull.  "Bless your little heart." is a classic way for women in the South to play a clear "you are an idiot" card with catty idiots.  Treat them as they behave. By choosing their behavioral crap, they are choosing the ass baring consequences.

Rock your happiness, bare their asses if necessary.

Enjoy your wedding.

This wedding is going to be a mindfield. I would talk to your DH and to SD about the various guests that are invited. Make yourself a mental roadmap of the people that you should try to hang out with and talk to. Are there any cool ants or family friends or something like that that you could hang out with. Sure there will probably be comments but if you surround yourself with other people that you are comfortable talking to then who cares what the other people say.

You have a host of problems that are far bigger than what to do at the SD's wedding.

The entire family sounds enmeshed. The SD marrying ex SM's nephew or whatever. You will now always have two BMs bubbling at the surface. Even WORSE and the biggest problem is your DH. He cannot say no to SD. He will go broke out of guilt giving her everything she wants, and to hell with what you think. BTDT

Yes SD will take away your thunder from your wedding, that is if she wants to try and ruin things for you. If she happens to be decent then ignore this comment. Please pay attention to my above two comments and what everyone else is saying.

You are not even married and all these issues are erupting. This is an appetizer to what married life will be like.

Does the SD know this guy?  It seems bizarre and ill advised to marry someone you've never gone on a date with.  So is he basically a stranger, or is he someone she knows very well as a friend and they've decided to take it straight to the altar without having a technical "date"?  I'm not sure why they couldn't just live together in Hawaii.  I can't imagine the spousal support and housing allowance will be more than the cost of the divorce if it doesn't work out.  I hope your fiance at least had a conversation with her on the merits of this marriage before forking over the $$$$ for the wedding.

As for the car, if the amount is more than what your household can afford, I would be pitching a big, ugly fit if he agreed to help her with it.  Big and ugly as in postponing the wedding, because that would be a terrible way to start a new marriage.  Now, if he can easily afford to help her, let him have at it, but if not, it's a huge red flag that you need to pay a lot of attention to.

I'd have a conversation with him before SD's wedding letting him know how you expect to be treated at the wedding (i.e., as his spouse and as one who paid for a nice chunk of it, meaning you'll be seated with your husband, you'll be included in pictures, you'll be treated with respect, you will be protected from his BMs if they do not act appropriately, etc.)

She is old enough to pay her own bills.  From her wanting to have her daddy pay off her car to the details of her relationship and getting married, she sounds way to immature for marriage.  

It sounds like there are a lot of issues here in your post.  You seem to be focused on how to feel at SD's wedding when I think you need to be focusing on your own and what exactly you are marrying in to.  This group seems to be too close and too chummy for my taste.  Your DH can't stand up to his adult daughter, you are paying for weddings and cars and you aren't even married yet.  I would think long and hard about putting a tent over that circus if I was you.  

A HUGE no. Just No. Complete sentence.

Id worry more about Disney Dad-Daughter wife if he cannot say no to SD, than the wedding with 2 Bm's,

Well our wedding ended up being amazing and SD was really so supportive and loving towards us she helped a ton! We discussed and worked out something that financially makes sense in regards to paying off her car.

So, looks like YOU overreacted.