In Mexico, there are two dates for “trick or treat”. One is for kids (the Day of the Holly Innocents All the Saints) and the next day is for Day of the dead or Día de Muertos, which is for everyone, in a clearly adult-centric celebration. The treats in the first day are candy-like, in the second day it’s very-Mexican-food-like.
Ask your sister which one would she celebrate. The rightest answer is both, the right is one or the other, the wrong is none.
Also, if she’s watching after some kids, that’s great and deserves a treat. Ultimately, as this post and comments suggests, it all depends on the people’s heart.