It does sound a little off to use you to get a discount without saying anything first. All I can say, is it has cost you nothing apart from the worry that they will ruin things for you by talking about it.
Just don't meet up with them and avoid looking at any messages until you have been. If you are on social media, just hide any of their content once they've set off on their journey.
Thank your lucky stars that they aren't going with you, that's what my Mum and Aunt did when we booked a holiday as a family.
Its not unreasonable to think they shouldn't have done it but it would probably be unreasonable to make a big song and a dance over it, especially if you take damage limitation control!