The mood of the interview may depend on how much her family knows about ADHD (ADD is no longer the label used by the DSM, and the variant of ADHD which doesn’t present with hyperactivity is called ADHD - Primarily Inattentive, which is what I and most females have (which is partly why so few girls were diagnosed with it in the 80s and 90s. Teachers and parents expected anyone with ADHD to be hyperactive, but really only the boys show up that way.)).
The thing is, ADHD is not an exclusively negative condition. People with ADHD tend to be bright, very creative, funny, a whiz at whatever subject has captured their ability to hyperfocus, and to work well in sudden-onset pressure situations. So, as much as the person and her family have to find ways to cope with the downside (impulse control, inability to focus on repetitive tasks, difficulty following a list of instructions. forgetfulness, distractibility, et cetera), there will be things that she excels at and her family will turn to her for without even realizing it’s the ADHD making it possible.
I guess it also depends on what you’re wanting to focus on in your interview. Do you want to explore how she is treating her ADHD? Some people don’t do anything, but most of us do something - dietary changes, lifestyle changes, counseling, medication, career choices, spoken and unspoken support arrangements with loved ones. Do you want to see how her diagnosis has altered the way she sees herself or how her family views her? I have close relatives who get that ADHD makes me bouncy and sociable and distractible on small scale stuff, but they’re still very judgmental on larger scale stuff - looking down on me for having less than stellar credit and such. When was the diagnosis, for that matter, made? If I’d been diagnosed in middle school - which is when it really started to have an impact on me - my parents would have known that I wasn’t being lazy or willful or immature, and that would have meant the world to me.
Once she had the diagnosis, what did the parents do about it?
The kind of questions you ask can redirect them from a bitch-fest and may even serve to educate them or let them know that they’re not alone. If you’re worried that the family members’ answers are going to be stressful to your subject, you might interview them separately. She’ll still read about what they think, but it’ll blunt things at least a little bit.