Dealing with your DM, she will probably have one of three visions in her mind:
-the non verbal end of the spectrum, which clearly your DN isn't, but this could cause denial and a well it's not from my part if the family sort of attitude,
-The genius, pianist or mathematician, , bullied but eventually wins through - where the worry is , do they?
- the geek, bullied, unfriended.
Instead, find some great role models for her, of people whom she'll have heard about who are successful in a range of ways.
And then say how brilliant it is to get a diagnosis now. ( it really is, I wish DH had had one as a child), because there will be more understanding and support at school and at home, ability to pattern the mind, train it, in coping strategies, at this age, and to realise you're not alone.
And really important to realise now, not as other pps have after many years, what the issue is, having gone through a lot if life wondering why things don't quite fit or why things are a struggle for you, not others.
Also point out to DN. as appropriate, the brilliant people who have been on the spectrum, that it often seems to take the level of focus that the spectrum provides to create works/activities of genius, and that other people have different neurological differences too. There's a wide range of brains out there and all are valuable.
But DN may need confidence boosting from time to time. You sound like the best sort of aunt, just the person to do it, without the direct baggage of family expectations. Take her out doing what she wants to do, in the way she wants to, only pushing boundaries a little. Be her safe place, her safe adult, where she can email or text you.
And be so grateful that she's in this country in this century, things are far worse in many countries and when DH was growing up, it is a long shadow that gets cast,