If you read the book that I suggest, it will lay this out as a sucker's argument. This is where the cast takes the bait of a stinky drain as a symptom instead of addressing the root cause disease of the non-functioning relationship between two (or more) people.
My guess is that the drain is not the issue here, rather the MIL needs something to control. If the laundry room did not "stink", then the bushes in the back yard, roof, or new paint in the living room would.
I use Muratic Acid in the front loader and dishwasher to control both lime and bacterial build up.