Oh gosh, I surely understand what you're going through. My father has had one medical crisis after another for the past four years... he takes a dozen medications a day, and is presently on chemotherapy to try to prevent a recurrence of any of the three types of cancers he's had. All those drugs, piled on top of age and pain, make him say things that are just devastating to my mom and me. Ten minutes later, he even doesn't remember he said it, and he's humming and making jokes.
What we've found is that it's better not to try to reason with him or calm him down. If he's made an accusation that we feel we have to respond to, we try to tell him as gently as we can that "No, that's not true, Papa"... and if he persists, we just distract him -- "Hey, would you like a cup of tea?"
It's a terrible feeling to have to humor someone you have such love and respect for... but the alternative is to end up arguing all the time, and that's not good for either of you.
And I think the most important thing is to be very affectionate. It's amazing how just holding someone's hand for a moment can calm and reassure him. And I make sure never to leave my father without telling him I love him. Even if he's angry at me, I go over and pat him and tell him, "I love you, ya ol' grump." :-)
I don't know what's wrong with your mom... I hope she will recover. But if she doesn't, you'll always be glad you remained patient and loving toward her, no matter how her illness made her behave. Try to remember that it's the pain, and not her real feelings.
All good wishes to you and your mom...