Am I being unreasonable???

[Chellefy]

I'm feeling really angry right now and I just want others opinions of whether I'm over reacting or not.
So my sister has 2 children and her boyfriend has 1 child. (they share the youngest together).
Anyway I booked a holiday for her and her boyfriend and the 3 kids as Xmas present and birthdays presents for the kids really (early ones).
So I then find out that she has changed the booking and now none of the children are going but she is taking her boyfriend's sister instead.
Now I've never even met her bf sister and I had booked the holiday really for the kids so I just feel really angry! I've ended up paying for her, her bf and his sister! Now I really don't have money to waste as I live on just carers allowance that my son gets and this was a way of doing all bday and Xmas together but it's obviously backfired.
Anyway less rambling but am I being unreasonable to be angry?

Did she want the holiday? I don't think YABU to be cross that she changed it without asking you but for me a Sun holiday with my kids would be more of a sentence than a gift. If I was asked beforehand I'd politely say I'd rather not go. If she gave you the impression she was really up for going away with her kids then she is definitely being out of order. If it was a surprise I think it was a risky one.

I think YAB a bit U. If you book a holiday for someone without consulting them it's a bit presumptuous. It clearly didn't suit them for whatever reason. So whilst yes she is being a bit cheeky you also made a mistake in your choice of gift.

She is a CF. Tell her you think so. I would.

Did u check she wanted the holiday for kids in the first place?

She will have to spend a lot of money while there. A lot more than the £38 the holiday cost. Is the BF's sister a lot better off maybe and they're hoping she'll pick up the tab for a lot of the travel/meals/entertainment?