I have 5 children, and I make them help out as much as possible. What really bothers me is the enormity of the task some days.
I get really frustrated when I think of the laundry, cleaning, dinners, and then the big things like sorting through clothes that have become too small, defrosting the freezer, cleaning the windows, the stuff that has to be put off til there's time (which there never is).
When I feel like screaming I make a list of TATT
or Tiny Achievable Tickable Tasks. So instead of putting down
Kitchen
Laundry
Sitting Room
Hoover
I put down:
Sweep kitchen
Clean surfaces
Tidy
Wash cupboard doors
Mop
Tidy sitting room
Hoover sitting room
Clear bookshelves of crap (this gets done rarely!)
And so on, so that I can see that although there is a mountain of work to be done, I have done something, and, bizarrely, it seems to help.
I also take a half hour break for lunch every day when ds3 watches TV and ds4 is napping, no matter what needs doing.
Having your sister over, so that she can see how busy you are is probably a good idea. I know mine was amazed at how little time I have, when she came to help out before ds4 was born.