Does anyone have a form letter we can submit all my MIL's bills to make sure all communication and bills go through the Financial POA?

One thing you can do is have all of your Mom-in-laws mail go directly to your Sister-in-law's house.... that way Sister-in-law can sort through the mail. It's too complex to try to have personal mail separated from business mail. Seems like businesses want you to create on-line accounts to make any changes.

My Dad did move, so that made it easier with the mail. I sent out change-of-address notes to friends/relatives. And with the bills, newspapers/magazines I went on-line, set up accounts, and did the change of address to Dad's new place.

There will be speed bumps with some on-line accounts. Like I have one saying that my Dad already has an account and he needs to use that one. Fine and dandy, but Dad can't remember his password, and if sent to this email, he can't remember his email password to get into his email to get that password.... [sigh].

Then there will be those bills where you finally give up trying to open an on-line account and you call them. Sure, they will send the information to the address they have on file..... whoa, stop the presses, Dad doesn't live there any more... then they say he needs to go on-line to get a change of address.... but we can't get into that part of the website without a password. Who's on first !!

Good luck, at times it feels like a full-time job.

^^^ I forgot to mention in my first paragraph to have the Post Office forward all the mail to SIL house. One form can be used for that, but Mom-in-law would need to sign it.

My Dad use to accidentally throw the bills out thinking they are advertisements :P

A copy of the court order should be attached to a letter or fax. If creditors call, say " I can send you a copy of the court order via email or fax" Fax machines are cheap and save a lot of headaches.