Fakku? Probably not since I just signed up.
Do they know I watch hentai? I'd venture to think so. As Emoroxas said, she knows I'm pretty weird. In fact, when I was younger, she walked in on me. Now, my mom, at the time, was a typical American parent. "Grounded, -insert time or class I need to bring grade up-." She could never actually restrict me from it as I always had alternatives to reading and watching hentai if I so wished. If she DIDN'T approve, I can guarantee she wouldn't approve of my use of leisure by watching anime or buying manga with my money. When I was younger, I had a lot of stress in school. I was barely able to make friends and my grades were down the shitter. So, she might have just approved for the relaxation masturbation brings or just because I enjoy it. My mom wasn't/isn't the type of person to restrict someone from doing something they enjoy doing. As long as I'm quiet and don't wake her up, she'd never give two pots.
But I'm legal now. Even if I still was in the house, what's she going to do? Ground me?
NOW MY DAD? He wouldn't give a flying fuck. In fact, I think he watched hentai. Shoo, he'd watch porn on the big screen in broad daylight with the blinds open and everything. I'd walk downstairs to fetch a bowl of cereal and have to walk like a monkey because of the raging boner I had. My dad was the type of person to allow my siblings and I freedom. My sister was a closet lesbian and watched porn all the time. She'd sneak my iPod Touch out of my room while I'm sleeping and watch all night long. I'd check it EVERY morning. I lose PC use for 6 months... But when she did it... Anyway, I'd always clear history.
Funny story, incase anyone cares. When my parents finalized the divorce, my dad gained custody of me. Apparently, under Florida law, if another should move out the state the child lives with the other parents. He was just moving in and we were pretty much done. Just placed the couch. The TV and boxes were set and everything. (My dad had DirecTV and Comcast. Something about how he'd get certain games on each -.-). Comcast was set up first, so I turned it on. My sister, dad, and I just stare and wait. "aawwhhh yeahhh mmm". My dad had apparently ordered porn. Welcome to the neighborhood.
CHO-KAY!
tl;dr Yes, they know. I'm the only one in my family (both sides) that watches even anime. I wouldn't even bet money that my cousins even know what "manga" is let alone "hentai". To the site? No.