Either she is maneuvering the situation so that you HAVE to take him to the Memorial, so that YOU have to show up, too...
Or she actually hates your father-in-law's guts, and can't stand having him in her vehicle. [Is he incontinent??]
I suspect that it's a very sneaky, dishonest way to get YOU to the Memorial - and she probably congratulates herself on her skill at "bringing you back to Jehover" , every time...