Even if your dh can't afford ticketsmaybe he could ask your mum how she managed to be so lucky as to get tickets as he has been looking but not found any and that it would be so fantastic to go and experience it all, soak up the atmosphere etc. (would he only want to go to stuff in the Olympic Park?) and that her tales of being there have made him even keener than he already was to try to see something.
Then maybe if she manages to get a third set of tickets she might consider taking your dh with her. And in the meantime your dh can continue to look for tickets and be unlucky in finding any reasonably priced ones. It might not have occurred to her that your dh wanted to go, but dsis might have been talking about it with your mum and then they enjoyed it so much they though they'd do it again.
Do you know if your mum treated sis or did did pay for her own tickets? Maybe she knows your dh can't afford to go and she didn't want to embarrass him by asking and him not being able to go or finding money when he shouldn't - or dsis paid for her ticket. and would your dh be able to go at short notice?
Of course it sucks if your mum does treat you differently - regardless of whether it is intentional or not - when it is something like this that is potentially a once in a lifetime thing and that means a lot to you. I wouldn't try to treat my kids exactly the same as they have different interests and like different things but I do try to make it equal in the end.
And I can remember my dad coming back from a trip abroad with a bottle of perfume each for me and my sis. He happened to know what her favourite was so bought a bottle of it for each of us - despite my mum pointing out a different one that I liked that was a different price. He didn't want to get it fore as he hadn't heard me say I didn't like the other one and I did like it - on my sis, but it wasn't something i liked for me. The one I liked was cheaper for a smaller bottle thant sis's or more expensive for same size but dad didn't want me to think he was buying me less or spending less and he didn't want to pay lots more for the same size. As it was I would have been much happier with a small bottle of what I liked and maybe a tonlerone thrown in for good measure :o because it worked out that I got nothing (didn't like the perfume so have it to my sis) and she got 2 bottles.
However at least I knew that my dad would have been really upset if he realised what had happened and he had made his choices because he wanted to treat us the same!