So his sister (let's say) comes on her own to your big house and uses it as a base to do other things.
She is in and out and makes plans last minute.
She isn't as tidy as you and does things differently to you.
But she will babysit for you occasionally?
To be honest - this is family, IMO. Although I think you are perfectly within your rights to limit the time to what you are comfortable with. However 4 hours travelling is quite a lot for a weekend.
If I were you it might be that I would stop playing host. No special meals for her - if she is in when dinner is being prepared then she is asked if she wants to join and extra gets cooked. You could say 'are you in for dinner tomorrow? If so can you pick up some bits at the supermarket?'
'Can you watch DC on Tuesday so we can have a date night?'
'Can you bring your cups straight to the kitchen?'
'Could you possibly load the dishwasher this morning, I'm rushing out'
It sounds a bit like your people pleasing is meaning they rather than making the trip work for both of you, you are not communicating what you need from her.
Don't worry about offending her, she'll either be happy with chipping in a bit more or she won't stay as long/as often.
You could even say to her to come as a guest for 3 days, or come as a member of the family for a week. That is her choice.
She is family, you have the right to communicate your needs with her rather than just keep quiet and smile as you would with a friend who you could just never see again if you didn't want to.