B.W.
There are a number of problems here -- First, what bank allows a minor to have a checking account with no adult on the account? Minors do not have the legal ability to contract, so they are not liable for any transaction they make on their accounts. That's the law. (I'm a banker)
2nd, if the bank returned the check to the person who wrote it, they would do so in her checking statement, which would mean it was PAID by the bank. The cancelled check (cancelled, meaning processed by the bank) is proof that the check was paid. Most banks do not return checks anymore, but store them electronically, and will provide a copy if she needs to prove that she got it.
A simple call to the bank should settle whether the check cleared or not. Generally, when it isn't paid, they bank will say it was "returned" and this means that there wasn't enough money in the account to pay the check, so the bank sends it back to the bank who presented it to them for payment, and that bank returns it to the person who deposited it -- namely the lady who did the jewelry party. (BTW, banks do not redeposit a check more than once. They send it through the system, and it may come back. If so, they can only send it back through one more time. After that the customer has to take the check and get the money from the person who gave them the check. And your MIL should have INSISTED on getting the check back when she sent the money order. Because the check is still legal tender if it is not returned to the maker.)
Anyway, Mom steps in, sends a money order, and it got cashed. That means that the money got there, and she can get proof from the issuer of the money order should she ever need it. I am assuming paying for the order isn't the problem ?
I am assuming that your sister in law hasn't received the order after collecting everyone's money ? (otherwise, there's no problem, right?) have her check the customer copies of the order forms, because there's probably a corporate number that can be called to complain about the service, and the company will want to see what it can do. I am assuming your sister in law has a cc of the order info ?
But, before I call the company, I would call the dealer, tell her that you have confirmation that the money order was paid, and that you want to know when the products will arrive. Keep track of the conversation -- the date, and what was said. Write it down. If she then fails to deliver, THEN call the company. I don't know if there's anything they will do to stand behind their good name, but if they won't intervene, THEN go to the authorities, with a copy of the order, a copy of the proof that the money order was cashed, and the notes from every conversation regarding the problem -- the calls to the dealer who did the party, and the calls to the company. Show in good faith that you tried to work it out, and the dealer failed to deliver. She should then be charged with having to produce either the order or return the money.