Coleoptera I have twins, one with ASD, both in first year of A levels, and I think sanity comes from giving up all attempts to "equalise". I've met many adults who talk about how much it affected them when there parents tried to make everything "fair" and downplayed their successes so as not to upset the others, and it always backfired. I feel that my twins are each successful in entirely different ways, sometimes overlapping in stealing each other's thunder sometimes just in different zones utterly. I have one sociable one, one not so sociable and I noticed that when the sociable one sees the unsociable one succeeding she is actually relieved but actually she is relieved because she knows I am less stressed, I take the weight of it daily and that is what affects her, not the comparison with her twin in itself.
I feel that it is very important to try and de-sensitise oneself as a parent and mother, because our children actually suffer a lot when when we are too upset or overjoyed by their success or failures. I know you felt in your heart of hearts that Oxford would really be the ideal place for DT2 and you wanted him to be happy so naturally it feels like a blow for him and YOU. He will be picking up on that. You actually need to set a boundary here for him to help him move forward and to reassure him. Children with ASD can be very set, and very black and white, and so you need to be very black and white about showing that you are happy for him to have attempted but not at all sad that he hasn't got in. I think just knowing your investment in his happiness is an added burden. When you can say by contrast, to a child with HFA, I know this seems difficult now, but you are going to be alright and have a great time at another university where the course will be equally demanding and suit you AND you will make friends. AND MEAN IT. And draw a line in the sand, I don't want you to continue to worry about Oxbridge, you learnt a lot from the experience and now we are booking an offer day at x uni. That is a relief for them. They have their own hopes and dreams but they are really affected by ours too.
Siblings in many families have differing results from Oxbridge, and even at two year gap the feeling of not measuring up or doing better than another sibling can be profound, but they have to manage it, or no-one would ever attempt anything and we would all end up like the Three Wise Monkeys. Yes it isn't fair, but life isn't fair and never will be.
Good luck with today but also Celebrate DT1's success if he gets it, and commiserate with him too if he fails, rather than feel relieved. And then move forward.
York is great for History BTW. My nephew went there very recently and said teaching has been really excellent. He did a MA elsewhere for location reasons and said he looked back at the teaching at York and was really struck by how good it was (in comparison to the second place)