I have a DC with profound SN which affect every part of family life. I also have a younger NT DC.
I would say definitely yes, allow your DD that resentment.
I think one of the most effective techniques to try is to allow your daughter all of her negative feelings. Let her tell you about the resentment that boils up, the jealousy, the dislike and the anger. Acknowledge that those are her feelings. By this I mean make sympathetic noises, nod your head, say 'Yes, I see that it really, really bothers you' etc etc. Or ask her to write a letter describing how she feels. Acknowledge that things are at times hard for her.
Do not show that these feelings upset or anger you. Allow her to air them. You don't have to agree with them (though with some you may want to). But don't come up with the explanations as to why your DS does a particular thing and why it's not his fault. It's not the time for that. Besides which she will have already heard a lot about that.
Let her tell you the feelings, give her an outlet for them. You don't have to agree with them and you don't have to accept her treating your DS badly. But let her know that her negative feelings are natural and allowed. All feelings are allowed, though all actions are not.
It's basically using the techniques in How To Talk So Children Will Listen And Listen So Children Will Talk'.
I think she will actually start to feel less resentful of how things are if you acknowledge those negative emotions first and foremost and keep on doing it.
I also think it's powerful to acknowledge how important she is in your family. How it's just as important to you that you are there for her too, not just her brother. That her needs and desires matter too.
I am not saying you have to take action and do everything differently at all, in fact this technique does not involve action as such. Besides, I have no doubt you are doing a fantastic job in v difficult circumstances.
I do think a good dose of sympathetic non-judgemental listening is amazingly powerful - it's bloody hard at first because it's very difficult to listen to negative emotions being conveyed and you so want to defend your other DC. But sympathetic listening and an acknowledgement that it's difficult for her too, could go a long way to making your DD feel less resentful. If she feels less resentful, she will act less resentful which means things will improve.