Hey there Zashira, and welcome to planDisney. I am very glad you asked this question because many people wonder how to add a Guest if a special event is sold out.
Mickey's Not-So-Scary Halloween Party and
Mickey's Very Merry Christmas Party are so popular that when the tickets sell out, they truly sell out. Disney is working hard to make sure the theme parks are not too crowded and Guests have shorter lines with a value-added experience. Is it possible that more tickets will become available for your specific night? Not likely, but you can keep checking the website. The data is in real-time.
Special Event tickets, like all Disney theme park tickets, are non-refundable and non-transferable but I have been able to move my party ticket from one night to another due to an unforeseen conflict. So you might be able to move your ticket to another night if the alternate night is not sold out.
Zashira, I hope you can enjoy the spooktacular events that unfold at Mickey's Not-So-Scary Halloween Party with your sister. Please do not hesitate to write
planDisney again with any follow-up questions or for more Disney vacation planning advice.
Enjoy making many Magical Memories.
~Jason