We're going along with it for this year anyway
That's very noble of you 
I think you need to get over yourself. You don't even have the right to be hard done by. Your MiL is only going to get worse and it's the most horrific illness.
Just buy the bloody presents to help. Or even better, tell him not to bother as he doesn't need to worry about things like this right now, which he doesn't.
And to all the posters saying it's ok, we thought he's asked you to do his washing or housework, again so what if he did?!
People are so unbelievably selfish at times. Is it really so bad to do the housework of someone who is caring for their ill spouse? Is it too much hardship to stick a load of washing on? To bung a few extra things in your shopping basket?
Being a carer for a relative is hard. I took on part time caring for my Dad, I did evenings, my Stepmom did days and my other siblings swanned around and did wonderful displays of public grief. They felt looking after him wasn't their responsibility as his child.
I emptied his bag, I wiped his bottom, I gave him his morphine regularly through the night, I held his hand while he cried in pain, I rubbed cream into his dry, tearing skin and I fed him. I also held his hand and told him it was ok and to go when he was ready and I sat with him as he took his final breaths.
And I didn't buy one sodding Christmas present that year as I couldn't cope with that.
Even after that exhausting 18 months, I still feel grateful he didn't have Alzheimer's as I think I would have struggled. My old Managers Mom has it and she gets progressively worse each day and has been like this for 4 years but is in otherwise good health.
So yes, you are being massively unreasonable but I bet your still offended at being asked.