She is mad at you for staying at your elderly mother to ensuring her safety? How ridiculous. So when and if you two move in together, she would expect you to never take care of your mother? That’s crazy

So there might be a couple of explanations here.

She’s maybe upset about something else but wouldn’t tell you. Was her first husband a cheater and is she afraid you are going to sleep with other women but lie you were at your moms?

Second explanation is that her true colors are showing and she’s not the kind of person you want to spend your life with

Try to get to the bottom of it.