Hello, just a quick homework question.

How do you say: working for my sister's buisness - is it sister's with an apostrophe s or just s or apostrophe after s?

3 answers

If there is only one sister, then use sister's (the singular possessive form).

If there are two or more sisters, then use sisters' (the plural possessive form).
Also -- watch the spelling -- it's business. You have the s and the i reversed.
Thank you very much