That's awesome! you should be so proud of your sister and brother-in-law for changing those travel plans and making sure the french knew why!
I am--and I didn't even have to talk to her about it. They had made the plans months ago--and I didn't know they'd changed their plans until they returned (it was one of those "Oh, you mean I didn't tell you? I thought I did" sister moments). I was overjoyed.
Just think: France lost out on the tourist dollars of four fairly well-to-do tourists (my sister, her hubby and another couple). And, knowing my sister, she was ready to spend freely. Lavishly, as a matter of fact. And that money went into the coffers of the UK and Ireland, not France. I hope more people emulate them.