Random thoughts:
If her diagnosis is recent, then she now has the freedom to stop hiding it - she now knows she is not just "thick".
She may be angry "why did you not know?" " you want me to be this way, but I cannot"
If she has a proper diagnosis, then the school have to put into place an adjustment plan to support her, which may have information to help.
If she is verbal and communicative, which it seems so, then she can be given behaviour boundaries and consequences. Just because she processes differently does not give her the right to behave badly. Her parents need to tell her this quietly and calmly and put her in a safe space if she acts out.
This is common to most 13 year-olds, but it may be that there are different ways of telling her.
They do need to get help identifying her triggers, and find out why she is seeking reactions from her siblings, what she hopes to get from them, and how they can help.
Just remember that she is not her label, is probably having a hard time dealing with the diagnosis, and is still an individual.
(I work in a class with 3 diagnosed children. One is extremely bright and knows how to play the system, one also has OCD and is "managed" by a series of bargains (if you sit still and listen for this lesson, you can have half an hour with the iPAD; you will have to do this test, so choose if now or during play time); the third is very quiet and withdrawn, and TA will sometimes take her out for quiet 5 minutes to calm her down).
Just one thing - it is more common than you think for multiple diagnoses, but often once the first is found the others may be missed - OCD, ADHD, claustrophobia, dyslexia, dyscalcula, mild forms of dyspraxia may not manifest as clumsy, but can mean poor or slow handwriting for example.
As to DGS, he may well be just trying to grab the attention back.
You could try some of the online awareness course yourself freecoursesinengland.co.uk/ www.futurelearn.com/courses/autism