I sense a recurring theme of reluctance on the part of DSIL to act on the advice she solicited. (If I'm misinterpreting this, then ignore the rest of my post.)
OP has suggested several options, but since the options involve making hard decisions, SIL isn't exactly chomping at the bit to adopt them.
I wonder if the difficulty might NOT be that the options are unpleasant, but rather that the suggestor of said options is too close to the situation.
How many times in our families and our careers have we seen solid ideas shot down because they were proposed by someone within the group? And then later on, the exact same idea coming from a hired outside consultant is welcomed warmly. (Grrr!)
Perhaps, when OP's patience is exhausted, she should consider engaging some third party to deliver the message:
- The family attorney;
- the branch manager from a local bank;
- SIL's landlord;
- a member of the clergy; or
- your golfing buddy who happens to be a stockbroker.
Any of these folks might seem more "objective" and therefore more persuasive.
It's amazing what you can accomplish when you don't care who gets the credit. Good luck!