This is definitely not as minor an issue as you make it out to be. It looks like there's a bigger overall problem that's brewing that has not been discussed. Unfortunately, I think that your niece's habitual lying might be a direct influence of your sister's personality, which is why your sister will never fess up to herself or her daughter making a mistake. Situations like this call for 2 things, a confrontation where you ask your sister to tell you of any issues she or her daughter are having, and an ultimatum, if your dog is left out one more time... then (fill in the blanks)
This is just my
