Question

what is the correct abc order for sister's sisters sisters'

Answer 1

Since the base word is "sister," I would put them like this:

sister
sister's
sisters
sisters'

But there's an easy way to tell if you have Word on your computer. Type the words in a column as I did above, and then highlight them and use the ABC button at the top of the screen. If that button isn't there, use the Help files (or press the F1 key) to find out about alphabetizing or sorting.