Sounds like your sister's party will be a lot of fun.
If your sister is an adventuresome person, I would suggest a game such as "Name The Guests."
In its simplest form, the host of the party/master of ceremonies tells your sister that it is time for the party games.
The host/mc then blindfolds your sister securely (be sure that she cannot see through or under the blindfold. If using a bandana for your blindfold, be sure to line it with aluminum foil as you fold it).
Have the guests stand behind your sister and make fun group photos. (Be sure to get at least one "fun" and one "serious" photo of just the surprise guests and your blindfolded sister. Similarly, be sure to get at least one "fun" and one "serious" photo of everyone including your blindfolded sister.)
Then the game begins. The simplest version of the game would be to have your sister name all of the guests at the party. Then, the host tells her -- you forgot some people... Keep going until she guesses that you are there.
Alternatively (or in addition) everyone could make their favorite recipe (if it is a pot-luck-style party). Your sister has to try all the foods and guess who made them. The "chef" for that dish then poses for a photo with her. The surprise guests remain out of sight (or your sister is blindfolded for the taste test). If/when she guesses that she is tasting your dish, the host/mc asks her how it got there. She has to figure out you are there.
For more ideas, please feel free to follow up with me.
Dr. Ronald G. Shapiro
DrRonShapiro1981@SigmaXi.Nethttp://www.EducationByEntertainment.comedited by DrRonShapiro on 2/6/2018