Singular
My sister has a car
My sister owns a car
(This is) my sister’s car
This car belongs to my sister
(This is) the car of my sister – correct but not usual
My sister has two cars
Proper Nouns ending with “s”
Jane has a car
This is Jane’s car
Marcus has a car
This is Marcus’s car
Plural
I have two sisters
They both own a car
They half own a car
They bought the car together
This is the family car
X: This is my sister’s car
O: This is my sisters’ car