I’ve been in a similar situation, but my youngest came back home with three children!
I may have had the odd moan to the other children over the years, but I would have been fuming if they had had a go at her on my behalf.
Your parents are not old, but your mum does sound pretty stressed, even so, as much as she moans, if she can’t cope, she’s the one who needs to say something, not you, not your siblings.
Lastly, regarding your sister, it doesn’t sound a very normal situation, I don’t get why she wants to be there all the time, something doesn’t seem right there.