cariboo, you have to face facts, you ought to have asked first if they were FREE to host you and your entire family for the better part of 3 weeks.
She'll have to get the house ready and clear up after you, so it will be more than three weeks of effort.
If your half sister said she wanted to see you outside the original dates then perhaps you could have thought to stay with or near her.. by the by.
And now for a For Heaven's Sake directed at you, PLEASE, PLEASE, PLEASE, learn from the billions of threads on here about the house guests from hell and don't just sit there and expect to be waited on. Your dad is 80, your step mum has enough to do, without waiting hand and foot on you and 3 step children.
That is asking way too much of even your own mother, let alone a woman you don't really have that much of a relationship with.
FWIW, when I lived abroad and had a trip booked for 2 years ago, my step father sent me a shorter sharper email than the one you got. I cancelled my trip, it ruined my summer, I missed my sister coming over from USA for a visit, I spent my 40th in the shittiest life imaginable, but I did not put my step father out. I told my mum and dad how sad I was, I told my sister all about it, they all thought StepFather was out of line, but I felt that I couldn't go after he'd done that.
You are bulldozing your way through your extended family, you didn't consult them before descending and you still have the poor manners to sit here and still not get it.
Make it your life's goal to organise little trips away from your SM house, use the house as a base only, and make sure that you cook, clean and help as much as you blooming will can. Your dad is 80, this could be the last trip to see him, so make it a good one.