Wills, no apologising! The whole point of this bit of the boards is that we can list out all that ails us and people will nod and think 'yup'. Lots of 'yup' to what you write.
We do have a completely different way of thinking. It's not based on feelings, it's based on pure logic, a bit like having your own Human Android in a way (though clearly we're human - I'm just trying to explain it differently!). If we understand the logic, we can do something. But then because we don't have a filing clerk in our brains to file away that amazing new information, we drop it into a big pile of random papers in our brain. And the next time the same sort of thing happens, we can't remember the amazing logic that solved it. So people have to tell us again...and again....and again...until there's so many pieces of paper on the floor with "Do this..." written on them that we can actually find one in the pile.
So, letting sister have something nice once in a while? It needs Rules. There are rules for when things happen. 'If someone has done things X Y or Z, then parent rule says they have a nice thing as a reward'. And each person who is with them has to say to them 'Well done' or 'you were very brave' or whatever it is and not say "I hate you" or "You don't love me" etc.
If the rule is broken, they don't get nice things for them when they do something brave, so it makes sense for everyone to obey the rule. Maybe be clear that if elder daugher can follow the rule for the surprise dinner, she gets something nice as a reward. Doesn't have to be money or gifts - we can feel really rewarded to get a bit of extra time on a favourite hobby etc.
Rules just need specifying really clearly as if you're teaching a robot how to do something. Takes a bit of thinking about, but it can work.
Even now as an adult, I yearn for people to just tell me what the rules are, then I'd know. Otherwise we get completely obsessed with ourselves because everything else is just too confusing and variable.
PS, visiting a restaurant is often as difficult for us as it would be for a fish which is taken out of its tank and put it in a restaurant chair. We have to try to 'hold our breath' long enough to survive the noise, flickering lights, smells, different tastes, social interaction, eye contact, sudden changes of plan...eek! Very difficult to explain this to people. You may need to let her take something very calming, or have an emergency escape plan set up for her if it gets too much for her, even if it's just ten mins outside at half-time?