Maybe she can become President.
If the OP does end up deciding to marry, I’d suggest he do a prenup and keep all of his finances separate from those of his wife. Just in case someone in the family makes a financial claim somehow on your fiancee after you’ve married.
Not a bad idea. My wife and I kept all our finances separate, because… laziness (and on my part, a deluded attempt to keep my independence).
For forty years, she’s been balancing her checkbook to the penny every time she uses her debit card, and I’ve been traipsing through life, assuming “I’ve got between two and three hundred in my checking, I’ll be okay…”
She’ll just shake her head and say “Glad I won’t be on the hook when you screw up.”
(In the OP’s case: “Glad I won’t be on the hook when you or your family screw up.”)
IANAL but keeping finances separate without a prenup does not get you off the hook in case of issues in the spouse’s finances.

And all of this is to say, you may need to ask yourself if all of this is really worth it. I know we want to believe “Love conquers all,” but frankly, that’s bullshit.
How long before we get a thread, “my inlaws changed the title to my house and sold it!”? “My SIL used my car to make a drug deal and the police siezed it.”?
Well, he definitely should get in the habit of just sending his fiancee e-transfers for shared bills. Frankly, even with someone I trust completely, I don’t casually let them take my frigging bank card whenever they decide to run errands.
…which is why they shouldn’t get married anytime soon. @Superdude needs to slow things down and first protect his finances.
I know this is going to throw a bucket of ice water all over “But we really love each other” sentiment, but he’s been violated. His fiance probably didn’t intend for it to be this way, might have had good intentions or at least not had bad ones. But @Superdude needs to take control of the situation.
Is she really marriage material? I think that will depend on how she reacts to these necessary changes. She needs to be understanding and she has to acknowledge that her family drama is a threat to him. It’s her job to protect him from that drama, just like it’s his job to protect her from any baggage he might bring into the relationship. If she can handle it, then she will have proven herself. If she can’t, well…

Well, he definitely should get in the habit of just sending his fiancee e-transfers for shared bills. Frankly, even with someone I trust completely, I don’t casually let them take my frigging bank card whenever they decide to run errands.
I gave my now ex-GF a Walmart Debit card to use for helping her out in paying bills instead of just handing her cash. I could put like $1000 on it and then set up a limit of how much she had access to. Worked pretty good, especially at the end of the relationship when I was able to pull my chips back in.
Well, I wouldn’t love that but it’s still better than your girlfriend whispering “I’m just taking your card to help with the groceries” while you’re sleeping for your nightshift.

(Though, how can Thieving Sis continue to deny it with her mom sitting there, having seen the evidence?)
Who ya gonna believe, me or your own lyin eyes?!
Reminds me of an incident some years back when a pretty well known member of a local college hoops team was arrested for a string or armed robberies. Let’s say his name was Robert. He was a LARGE man. When attempting to rob one business, the clerk recognized him and said, “Don’t do this, Robert!” The genius’ response was, “It ain’t ME, man!”

< snip > His fiance probably didn’t intend for it to be this way, < snip >
It’s fiancee, for women and fiance for men. (You can add the é [diacritical?] mark if you want to be absolutely correct.)
Sorry for showing my pedant.

You can add the é [diacritical?] mark
Option+e+e on a Mac, ALT+0233 on a PC.

It’s fiancee , for women and fiance for men. (You can add the é [diacritical?] mark if you want to be absolutely correct.)
If he’s smart, it won’t be either.
You showed them the truth; and then she still tried to squirm out of it. Butter would not melt in her mouth.
Worse, no one backed you up or co-signed a loan for her theft even when shown proof. This sh-t could be put on a VISA or pulled from a 401k, but it wasn’t. Time to kick this family of thieves to the curb. Its put up or shut up time… but hey don’t ask me. Ask your attorney and see if he/she tells you any differently. Also, pay to have a background search done on these Dillingers. You might just be surprised.
I’d love to say that it’s time for Cheesesteak’s ‘scorched earth’ plan. Go full GoT on her… and if you don’t get back every single penny plus interest, perfect a mortgage on their house. You never will, but it’s nice to think you might.
lol, pedantry appreciated - I had the feeling all along that one ‘e’ didn’t quite seem right but too lazy to check.
I would think the opposite. A prenup is pre-approved separation agreement basically. Keeping your finances separate is more important if you don’t want to be dragged into your spouses financial obligations. IANAL either though.
“My inlaws took out a million dollar insurance policy on my life and…”

I’d love to say that it’s time for Cheesesteak’s ‘scorched earth’ plan.
Technically, I didn’t actually recommend this, but just noted that any persistent “police based” response to this would be perceived as such.
I still don’t think he will get his money back, but with the mother’s backing, SIL may be put in a position to do something.
Still, this is the second felony directly attributable to bad acts by fiancee’s sisters. At this point, I’d recommend a zero trust policy for her siblings. Wallets in lockboxes, no sharing anything like computers or phones, nobody left in your home alone.
I am curious how fiancee went from “little to no contact with her family” two years ago to:

I suspected her simply because she’s the sister that is the most frequent visitor to our house. Fiancée has 5 sisters, but this is the one who she sees most.
Like, did you witness some family reconciliation or did you just slowly start to notice they actually have lots of frequent contact?
[This is Not a legal opinion] “Maverick…! You’re in her jet wash. Eject…! Eject…! Eject…!” [/This is Not a legal opinion]

I thought he meant same fiancée, same family, but the stealing sister this time was not the carjacked sister last time. I also don’t think the stealing sister this time was the druggie sister last time whose debt problems triggered the carjacking/kidnapping
Can’t keep track of the felons without a scorecard! Let’s see, 5 sisters? Only 3 more to hose you over! Either way, time to bail on this family. Sorry, Dude.