Devious SIL or....AIBU

There have been comments about your DH's behaviour. Head in sand? Quiet life? Letting her get on with it?
Frankly if your DH is consciously choosing to follow this course of (in)action he is actually complicit in SiL's actions.
If he ends up in trouble it will be his fault.
He won't be able to ignore it then and he'll have to face the consequences.
Time to get a grip and face up to his responsibilities!
(Possibly with the help of a well-placed boot up the backside from you!)

mcem metaphorical I hope
or perhaps not!
He's being an ostrich.
Backside is in a strategic position

jalima

When my daughter-in-law’s grandmother died they found that a daughter who lived with her had been spending her mother’s money. The bank wouldn’t do anything, as they said that the fact the daughter had her mother’s card and PIN showed that she had consented to the money being taken out.
I’m sure it’s different if the account holder has died.

Well yes Jalima really strategically placed!

I'd notify the bank immediately because that's the right thing to do here and nobody needs to know. I wouldn't want to get into arguments with the family for doing the right thing. FIL may need the money later in his life and his interests to me would be paramount.

I agree with JS06, the bank should be informed, its fraud.