What is the correct spelling of the plural possessive form of sister and business?

The plural form of the singular noun sister is sisters.

The plural form of the singular noun business is businesses.

To form the possessive of plural nouns that end with an "s", add an apostrophe after the "s" (s'):

The plural possessive for sisters is sisters'.

The plural possessive for businesses is businesses'.

Examples:

My sisters' names are Linda and Lois.

The businesses' windows along Main Street are decorated for the holiday.