What if they catch you in a lie?

My MIL has Alzheimers. I'm wondering if my problem is uncommon. I read that it is best to "fib" to keep patient calm and happy. My MIL has "called me" on a fib and gets very angry and distrustful. It is hard to know how much forgetting is real and how much she is "faking it." I am afraid that if I tell her her mother is away for the day (in response to constant "where is my mother?") she will say.........."she's dead isn't she? And you lied!" Anyone else have this problem?