You need to study how the possessive is used in English. For a singular noun (like "sister"), we add apostrophe s ("sister's"). For a plural noun ending in s, we just add an apostrophe. (If you had more than one sister sharing a room, it would be your sisters' room.)

Is sisters singular or plural?

“Sisters” is already plural; there's no way of 'repluralizing' it.

Does my sister's need an apostrophe?

my sister's friend's investments (I have one sister and she has one friend.) my sisters' friends' investments (I have many sisters and they have many friends.) my sisters' friend's investments (I have many sisters and they have one friend.) my sister's friends' investments (I have one sister and she has many friends.)