Normally, I would not recommend this in the name of freedom of religion. However, I no longer believe in returning good for evil (it simply leads to evil being used as a "work order request" for good, not in your receiving good). Your religious freedom has been violated by such blatant guilt trips about the boasting sessions and the humming of Kingdumb Maladies just to pxxx you off.
And now it's time for paybacks. A single warning should suffice that the next time your mother hums or sings (or plays) Kingdumb maladies around you on purpose with the idea of guilting you back to the cancer, you will start humming, singing, or playing Christmas carols around her on purpose so she can have those songs going in her head. And, if the blatant preparations for boasting sessions with the intent of imposing a guilt trip persist, you start blatantly preparing for Christmas with the whole nine yards on decorations. With the Christmas songs coming up in a few months anyways, that's probably the last thing any witless wants--a head full of Christmas songs in the middle of September or October.