L.W.
I guess it would depend on your level of comfort with SIL and Hubby. Obviously being an "outsider" makes it a sticky situation. Maybe doing some research on her (MIL) specific diagnosis and obtaining some literature on it that could be given to them may help. You could say something like "I came across this and thought you might like to read it." Sometimes people just need the facts in black and white MINUS opinions. If that doesn't work and you are able to take the direct approach without a blowup I would just do it. I don't know a whole lot on memory issues other than it usually gets worse. The negative reaction from MIL you have observed can't be good! I sympathize for you....