Granddaughter upset

I wouldn't go in there and start an inquest on what has been happening. Have some quality time which doesn't mean spending large amounts of money. Time is more precious than anything else. Is there anything she enjoys doing? If there is a park or leisure centre nearby perhaps you could spend some time there. Have lunch somewhere she chooses. If she wants to chat then listen. Don't judge or blame anyone for anything. Let her know you are there for her and most importantly, loved.

You dont say what problem she has or how you know about it, and may have been resolved by now.

You say that you are not close now so doubt she would welcome your input anyway.

Your OP is not clear.
You say her brother has ADHD and then ask what to do about your GDs problem.
Why do you think you need to do anything.
And about what ?

She might have dealt with the issue by the time you visit. I wouldn't ask her what the problem is but let her tell you. Maybe find an opportunity to get her on her own- car journey is good. Just listen to her. Sympathise but don't criticise either parents or her brother. Ask her what she wants to do about it. Ask her if she wants advice. Do not tell her that you will keep what she says a secret. It might not be a child protection issue but it just might.

Sorry GranJan60 - I must be a bit thick today, but I can’t quite grasp your problem. Are you asking for advice on what to ask your GD? What problem do you think your GD has got?