Hi Elizabeth!
Thank you for bringing your question to the Disney Parks Moms Panel!
Mickey's Not-So-Scary Halloween Party is one of our favorite events and I love that you're dressing as the Sanderson sisters! You will definitely want to see the
Hocus Pocus Villain Spelltacular during the party. I recommend getting there 20-30 minutes early for a good view.
Here is a
link to the rules and guidelines for costumes at Mickey's Not-So-Scary Halloween Party. Here are the two rules you'll specifically want to take into consideration:
*Capes may be worn if the length does not go below the waist.
*Costumes must be family-friendly and may not be obstructive, offensive, objectionable or violent.
I think the zombie make-up should be fine as long as it isn't too "offensive or violent" looking. For your capes, I would try to keep them as close to your waist as possible. If they fall to your knees, security may possibly ask you to take them off. Obviously, each case is unique and it might be fine at that length, but I wouldn't want to take the chance of having to change my costume upon arrival if I can knowingly avoid it.
I hope this helps. Please let me know if you have any other questions and have a not-so-scary time at your party!