hello
@RL3
a warm welcome to DTP
sadly, what you describe is not uncommon ... dementia reduces so many faculties, one being empathy and another being reasoning
it's often the case that a person is most awkward, indeed nasty, to the person closest, providing regular care, and for your MIL that's you it would seem ... I think, the person 'feels' that something is not right with them but isn't able to grasp what it is, so projects the discomfort on, 'blaming' someone else for doing something that has made them feel bad ... and if there's actually really nothing wrong, the person will confabulate something to fill the gap, accusations which upset the innocent party
your MIL isn't directly meaning to be nasty to you, she's trying to make sense of a situation that makes no sense to her
unfortunately, caring for someone often means growing a thick skin, even apologising for things just to keep the peace (and not reinforce negative feelings) ... hopefully others grasp that you are doing a great deal for your MIL and that what she says isn't actually how it is