Is adhd a mental illness?

I got into an argument with my sister this weekend because she keeps insisting ADHD isn't real, that it's just an excuse for being lazy and disorganized. I sat there trying to explain how much I struggle every single day, the missed appointments, the forgotten bills, the exhaustion of masking all of it, and she just rolled her eyes at me. It made me feel so small, like everything I go through every day isn't even valid. Now I can't stop questioning myself: is adhd a mental illness, or am I just making excuses like everyone keeps telling me? How do you all deal with people who refuse to take it seriously?