A bit of background, DS is 6 and we've been going through the assessment process with him for nearly 3 years! We've had several stints of "wait and see" but as he gets older it's becoming more obvious that he just isn't quite like other children his age.
We also have an older DD 8, who is finding DSs behaviour hard to take, especially at school. She knows that he's been with us to a couple of appointments but we've not gone into it in detail. Just told her we have having problems with A's behaviour and are visiting X to see if they have any ideas to help us.
We've tried not to get too specific with anyone until we actually have a diagnosis. In many ways I expect we'll keep it that way, it seems better to just explain he has difficulties with XY or Z and we handle it like this. However I'm feeling we may need to explain a bit more to both children soon.
How have others gone about telling the child and their siblings? Any books (especially for the oldest) which people think are useful?
That is exactly what we did. There used to be a child at our very small school that I highly suspect was ASD. The family was not pursuing a dx at all but there certainly was challenges the child faced. DD1 was familiar with him obviously and when she started having difficulty with some of DS1's behaviors (he is ASD), we started having conversations about the other child she knew. She told me that the other child had "brain issues and that was the reason he acted the way he did." Because the student never had a dx, there never had been any discussions at school about him, this is what she had decided in her own mind. And to DD1, his "brain issues" was just like her having "learning issues" (severe dyslexia), it was just something that some children/people have or struggle with but others do not. It isn't always fair or easy to deal with but the way it is. She would get frustrated with other student just like she would with her own brother or her own SN. We allow a safe place to vent at home and she is old enough to help come up with creative solutions to some of the behaviors that DS1 does that do greatly affect her. She sees a therapist here and there and I know some venting goes on there as well.
I'm sure (OP) that there are things your daughter has trouble with. Whether it's Language Arts, or Math, or physical activities... Her brother also has trouble - with controlling his behavior. And where she is good at... whatever she is good at? Bro is good at what he's good at.