@kmakm
Oh Kate, take a deep breath my love...and then let out a huge primal scream!!
In regards to the exercise, do what you can, when you can, even if you only use it as a way of expending frustrations! I do think the current 'guidelines' are just over the top for a lot of people. The one thing i do remember when i went to the conference last year was when the guy from WA was talking about the quantity of exercise and he said, anything is better than nothing, even getting out for a 5 minute walk to let off steam. But i understand why you are sinking under a load of crap cos everything always comes at once and it's so much harder to deal with.
The cooking duties could be parceled out to all as it sounds like the teenagers are all old enough. Not fair that you are doing lions share.
You must keep prioritizing yourself because that's the only way you are going to be able to cope so don't give up on any of the me time.
The behaviour of your niece and nephew, that's a tough one. We don't have any contact with our 19 yo nephew since things blew up between him and my eldest sister (crashing her car in someone else's fence didn't help) plus it had been a toxic relationship for a long time. All you can do is be there for them. One day they'll thank you...i hope...they'd better!
In regards to your last paragraph, 'is it actually possible to do at all'...
All at once? Hell no...and it sounds like that's what you're trying to do or want to do. You can't, Kate, you'll only self implode.
As a lot of people have said take it all one day at a time and only do what you can and breathe...breathe...
Big hugs, Cath xx