Generally speaking, a gift is not required for an office Christmas party. The company is having the party for the employees...at least in theory. If there is some special requirement, such as a "Secret Santa" gifting, it is up to the person in charge of the party to communicate it to the prospective attendees so that they can decide whether or not to participate.
You should prepare your sister a Christmas gift that is tailored towards her likes and her interests.
Small trinket boxes are a gift that can be given for any occasion or party. If given at an office party it is still a tasteful gift. Some of the recipient might not appreciate them but that does not mean that it is not a tasteful gift.
Cash
Certain stores such as GeldWrappers or Amaze are gift designers who post ideas on how to decorate the home and office during special occasions. The catalogs posted on the stores have great suggestions.
If you been dating her more than a couple mounths . Then you should get her a gift
gift baskets or goody bags
the doll company the toy company
How about a gift certificate for iTunes?
Dirty Santa
gift cards always work
yes because it is your gift and you should be able to open the gift unless it is Christmas the you should wait until it is your turn
There are a number of places one can purchase Christmas gift ties. One can buy them from Amazon, ebay, Office Depot or Walmart. One can purchase them online or in stores.