It really doesn't help matters for people to keep referring to it as 'just a football match' that can be watched on TV, certainly not to the OP or her sister as from that point of view it does appear like the guests don't give a crap.
But, I don't think it's as simple as that. I am sure they do give a crap, it's just extremely unfortunate timing. I don't know who they support but a quick Google shows this is the 1st time Sheffield Wed have been to Wembley since 1993 I.e. for the 11 year old it hasn't happened in their lifetime before, and if it's another 23 years then they could well have kids of their own by the time they get this opportunity again.
Of course if it was just a football game it would be extremely rude, and in fact it is rude still. There's no way of getting away from that....it is rude and I do think they should pay your sister for any costs.
But please believe me when I say this is unlikely to be about not giving a crap about you or your sister, it's just what an incredible opportunity this is. This may be the only time in their entire childhood the 11 year old gets this chance, it's a dream come true for a Sheffield Wed fan (and hull too but they've been to Wembley much more recently) and is probably something they'll remember for the rest of their lives. I'm sure in other circumstances they wouldn't be so rude.
Now people may not and don't have to be football fans, but if you had the opportunity to do something that you hadn't been able to since 1993 that meant a great deal to you, something you could share with your child, and don't know if you'd get this opportunity again, then in my opinion that puts a different slant on things.
Like I said earlier, I don't blame you or your sister for being upset and I agree it's incredibly rude, but in the context I've described I hope your sister can at least understand a little bit why they've done what they've done. I hope your sister has a great day xxx