Thank you for bringing your question to Just Answer!
I'm sorry to say that you may not like my answer. While I can understand why you're upset that your family holiday will be ruined, the illness of your husband's sister does take precedence over a holiday. If her illness is severe enough to warrant them going to Paris, then your husband did the right thing by making sure his father could be there as well. It's quite a lovely gesture, in fact. It may be the last time the three of them are able to be together. However, that doesn't mean that you're not entitled to your own feelings about it.
I think there may be several issues going on--unless his sister may be dying, three weeks is quite a long visit. (If she is in danger of dying, then 3 weeks would be reasonable). I can imagine it will be stressful being without your husband for that long.
The other thing that may have you feeling more vulnerable is the impending 'loss' of your son to boarding school. You would naturally feel some pressure to build some good memories before his childhood has passed by. However, you do have the gift of three weeks alone with your son to spend some quality time together. Bonding emotionally and going on a few outings together will give you time to connect with your son that you wouldn't normally have.
Since they are already gone to Paris, there's nothing to gain by continuing to be angry about your husband's use of the money. Instead, turn your thinking around to realizing you have 3 weeks to make some mother-son memories. Rather than focus on what you won't have (family holiday), put your focus on making these three weeks a time to really connect with your son. Make weekend outings together--mini holidays for the two of you.