According to Orthodox Judaism, there is no such thing as "half-Jewish" or "culturally Jewish". You either are or are not. Judaism goes through the mother, so if your mom is Jewish, your aunt's family will view you as being just as Jewish as they are, regardless of your upbringing.
I am sure your aunt knows your background, and will do her best to make sure you and your family feel comfortable. Here are some tips that will help you:
What to wear: A skirt and blouse or sweater that is not low cut. Your aunt will probably be wearing a skirt or dress that is long enough to cover her knees (even when sitting), sleeves long enough to cover her elbows (even when reaching up) and a neckline that covers her collarbone.
What to expect: The seder can get kind of long, and there is a lot that happens before the main meal. So make sure to get plenty of rest earlier in the day, and eat something before you go over. The point of many things in the seder is to prompt questions, so ASK whatever you want to!
Other things to know:
1. Many people like to bring "hostess gifts" when they visit, but this can get complicated for an Orthodox Jewish family, especially on Passover. I would say to either skip the gift, or drop off flowers on Monday BEFORE SUNSET.
2. Orthodox Jews do not use electronics on holidays, so you should turn you cell phone to silent before entering the house, and try to remember not to flip the light switches (like if you use their bathroom, leave the light on).
3. Orthodox Jews generally do not touch members of the opposite gender, so don't be insulted if your uncle does not hug you or shake your hand.
Hope you have a wonderful time! If you have more questions, please post them - I will try to be on the lookout for you! PS. Check out www.aish.com for lots of great background info on Judaism, Passover, and the Seder.