On top of changing the locks, to really increase the security at their house, they would have to completely ostracize and ban SIL from entering the premises. I am pretty sure they aren't willing to make that move just yet.
I understand this. I wish that non-immigrant families and communities had 1/10 of the group closeness that many immigrant families have. Some do of course, but it is rare.
Probably your FIL/MIL keep cash and valuables around the house. This can be halted. There is likely a bank within a few blocks of where they live. Have them open an account. Their daily cash needs can be met with money kept on their persons, not in a purse, but in a billfold, money belt, bra, or whatever.
If they have valuables, put them as well as deeds, family papers, and social security cards in a deposit box at the bank. If they have larger valuables or heirlooms, look into selling them.
As far as "this will only get worse", I think that is very hard to predict. She may be early in her descent, on the other hand she may be very close to seeing the light.
As to the miscreant and her family, IMO it has to be treated as their business. The husband seems to me to me acting in a remarkebly passive manner. If I had a wife staying out all night she would be spending all morning waiting to seen in the local county STD clinic. There's more than one way to spend a night out, and more than one way to finance gambling losses.
One other comment I have. When faced by these disruptive situations it is very tempting to fix on one or a very narrow range of possible paths. One example I have in mind is your attempt to decide if there are possibilities of violence, or if "This is just friendly low key neighborhood and family games". IMO, this is just too much money to assume it is all in fun. Get away, and stay away! She has a husband, let him deal with it.
Ha