My parents, who firmly disliked anime at first, were NOT impressed with the convention the first year. There is nothing for them to do there, no quiet place to get away. They actually ended up leaving and going to the mall (at the time, my sister and I were both in high school and old enough to be left there with our friends). Fortunately, they've let us go every year for...oh gosh, five or so years now in a row (I've lost track how many years I've gone!). They just tell us to find a ride. Of course, now we've both got cars and anime loving friends and our parents do not have to take us. :3
So, as far as your parents go, they will get bored FAST. If they're not really into anime, they may find the insane crowds and noise a bit annoying. Also, you will probably not be able to be with your sister all the time...so your parents will probably have to be there with you guys. I'm not sure how old you are, and if you stay in the hotel, you most certainly will be safe (SO MANY PEOPLE THERE lol). But your sister will need someone with her because it is very easy to get lost. I would talk to your parents about how they feel hanging out in a hotel full of crazy cosplayers (referring to myself as well xD). Because a few hours is NOT enough to see all there is to see at NDK.
