My MIL is terrible for this. She can't resist a "bargain" and buys all manner of short dated and broken
"treats" crap from the man up the market, passing it on to us. This culminated in her giving us 2 catering size bags of those horrible pink wafer biscuits, and I just told her to put them straight in the bin and if she didn't they would go in the bin the moment she left. She still does it now, but not quite as excessively.
Luckily mrs trb agreed with me but didn't want to offend her mum over it, so she just hid!
I also once made the MIL put the 2nd pack of 18 yorkshire puds back in the freezer. She was about to cook 36 yorkshires for 5 people - 1 of which was only 4!
So back to the OP, You need to take control of the situation, but if you're working against the mrs then I wish you luck. My kids are still eating christmas chocolate, but they only get it for desert, after their tea