Not parents but I just accidentally came out as an NB person (also adult) to my brother yesterday. He had a hard time with me coming out as bi, which confused me, but it's been nearly a decade since then. I told him I wanted to change my name and he was very confused; I mentioned I wanted something gender-neutral, and he asked if I was nonbinary.
I think he was skeptical, and surprised - he'd lived with me since I was a child, surely if I wasn't cis, he would've known, right? But his partner helped him to approach it with an open mind and to be supportive. Overall, it was way easier and more inconsequential than I thought; at the same time, I'm thrilled to be able to talk about my life authentically, with him and around him, and not worry about every word.
If you want my advice on difficult conversations like this? If it's safe, do it during a long car ride. It's a situation where the other person really has to sit with what you're saying and work through it. Obviously it can cut both ways - they're stuck with you, but you're also stuck with them - but in my experience, it's been great.