OP, did you change your PIN? It seems to me I was at the bank one day without my debit card, but I had my statement and consequently the account number. In the lobby they input my info, I typed in my PIN, and voila! I may have had to show them ID, though I can’t remember.
Set a thief to catch a thief—if you aren’t one, you probably won’t think like one. It seems critical to figure out the truth before proceeding, but I wouldn’t know how to trust my judgment. It would be nice if you had someone at the meeting (who was not in that family) who could compare notes with you afterward for an objective take on things, at least.
It seems likely to me that FSIL has been in trouble before. If sister A gets in legal trouble, do the parents tell sister B? It probably depends. If sister A,14 years old, gives in to a dare and gets caught shoplifting, do parents tell 4 year old sister B? If sister A goes to counseling, gets help, seems to be back on the straight and narrow…do the parents sigh and decide never to tell anyone, because they want their daughter to have a normal life?
Unless I missed it, OP hasn’t stated that detail—fiancee’s parents, alive or dead? They may have taken the secret to the grave. But I can imagine scenarios where fiancee truly didn’t know, yet someone in the family did. OP, have you tried googling FSIL’s name?
And I come back to the future situation: if you marry fiancee, things will change. If FSIL isn’t invited to the party, are sisters C, D, and E going to pressure Mrs. Superdude? Not this year, probably…how about in three, five, or ten years? Do you think fiancee should cut off ties, or is it ok to see her but not in your home, or…? Do the rules change if you marry?
Years ago I would get cash at an ATM in a gas station. One day I went to get cash and I had no money in my account. Someone had put a device on that ATM that captured the info and used it to take out something like $60 at a time every two minutes until it was rejected for NSF. The bank suggested I set a limit on how much could be taken out in a day, which I did.