In general, NDK is a family friendly convention, but how your parents react really depends on your parents. How are your parents with your interest in anime? Do they support it or make fun of it? And even though NDK is "family friendly" there are some aspects that strict or I'm sorry, I can't think of a better word, close-minded parents may find offense. People cross playing ( that is cosplaying the opposite gender) people of the same gender hugging or kissing, girls in skimpy cosplays like Yoko from Gurenn Lagen, etc. While overly explicit stuff is generally avoided, it really depends on your own threshhold. Also be warned that many people at conventions can be loud and immature. People yelling things like Marco polo, you just lost the game! and Buttscratcher!! Tend to be prevalent.
As for your sister. I'll say this, I REALLY REALLY don't think that NDK is a place for young children**. While it's family friendly, there are no events for young children. The majority of con goers are 13 and older with a growing population getting into their mid-20s or even older. While there are young children there.. babies and even toddlers, they tend to be the children of older con goers.
** However, if you're sure your little sister or sibling would enjoy what the convention offers: Video rooms, AMVS, dealers room, panels, cosplay and photos.. that's a different story. But asking if there would be anything for them to do has to be answered with: What do they LIKE to do? There's no daycare at NDK. Most anime shown is in the PG-PG-13 rating and perhaps at late night R rated, hentai and adult material is avoided. The panels may interest them and they may not. It's really a case by case basis.
Anyway here are my tips!
1. Bring a camera! There are soo many fantastic cosplays! Just remember to ask before you take a photo. If someone refuses, don't take it personally.
2. Read and follow the rules.
3. Make sure to pick up a schedule to plan out your day! Things like panels, video schedules, locations etc can be found there.
4. Remember to eat! Since your parents will be there, you may be safe.. but a lot of people try to get by on pocky.
5. On the same vein, drink water!
6. Will you be staying in a hotel room? If so make sure to get plenty of rest.. also SHOWER. Shower often. And reapply deodorant through out the day. You want to avoid the confunk. It's crowded and can be hot, you won't believe how fast you can start to smell.
7. Don't be afraid to say hello to the person next to you! In general, people are friendly! Say hi, compliment a costume, etc!
8. Have fun! XD