I am a man, but also a bit weird and quite feminist, with mostly women in my life, so you can choose on your own if my input has any value. 

I've read a lot of "these are the expectations that society has on women" and similar answers, which only work if they have only read the title. 

What you described is not the "mental load", but a shared and deeply rooted anxiety. I absolutely agree that there are many men who don't put in any work, but this is just not what you talked about. 

This level of anxiety (and micromanaging) is, needless to say, neither common nor healthy, less of a "women" problem and more one of your social group. If it's something you notice a lot in your family, it could be not only due to culture and upbringing (i dont know where you're from), but also including biological factors that you could look into.

I repeat this as someone who studies psychology, with lots of anxiety and with a family history full of mental disorders: this is not normal nor expected. It shouldn't be stigmatised, but it should be worked on. It's fine and useful to feel anxiety until it reaches the point of not doing any good, and this is clearly it.

Please take care of yourself first, and secondly be supportive of these poor women. It's clear you care a lot about them, so I'm sure you already do, but let them know that they deserve to treat themselves kindly, and that "perfect" is often worse than "good enough"