Am I overreacting to be so cross with my sister-in-law?

I would be furious if I were you. You planned this trip to be nice to her and then she pops up with this "surprise" so now you have to adjust all your plans to suit her. I mean its nice that she did something nice for you MIL but she could have had the decency to tell you that before you booked your tickets. I definitely wouldn't change a thing