This is absolutely typical. My aunt, who had Alzheimer’s, would tell my mother every week that she hadn’t seen her daughter for months and didn’t know where she was or why she wasn’t visiting. This at a time when the daughter and her family spent most of every Sunday with her and took her out for lunch practically every time! In fact when they’d been out and had lunch and taken aunt back home, she would almost immediately ask whether they were going out for lunch today! So please don’t worry about this with your MiL. To coin a phrase I often used in my working life, don’t look for logic where there is none.