Why did this happen and are we right to be cross?

I don't know your family, but I've seen it before. Issan girl marries a foreigner and thinks she is better than the people she was working with just the week prior. In Thai language/culture it is a very fine line between her talking down to the hotel staff and being friendly. My guess is that she was a bit uppity and the staff threw it right back in her face. You need to deal with your daughter-in-law, not the hotel!