How do I handle this one? MIL related

I am so aggravated right now. My MIL is crazy! She goes away on trips all the time and we end up watching her dog, who is 14 years old. he had 2 accidents over the weekend, and we were walking him every 2 hours except at night and that was every 6 hours. We are in no shape right now to be picking up carpet, even though we will eventually. I am glad we got it out of the rug. While she was away for 5 days in las vegas, we were stuck at home walking the dog for 5 days. DH took off a day from work , we did not go out saturday and on Sunday we saw my sister and BIL for 3 hours, it was my sisters b day. She had the NERVE to tell my BIL we must have not walked him as much as we said, probably 8-9 hours and it is our fault, she is in the land of denial. I love animals and I would never lie, i tell people like it is. The problem is that my FSIL told me this in confidence. What do i do, DH does not even know his mom is lying to make us look terrible. I am FUMING!

alot of times pets dont behave the same as when their owners are home
My mother's dogs seem to notice that she is gone and they react to it.
one will refuse to eat the moist food
and the other will pee all over the house...marking his territory.
both behave like angels when my mother is around.
so its possible that your MIL's dog is simply reacting to her leaving