The singular possessive form is stepsister's.
Examples: This is a gift for my stepsister's birthday.
The possessive form of the singular noun stepsister is stepsister's.Example: I'm making a cake for my stepsister's birthday.The plural form of the noun stepsister is stepsisters.The plural possessive form is stepsisters'.Example: The wicked stepsister's taunts made Cinderella sad.
"Firefly's" is a singular possessive noun.
The singular possessive noun is stepchild's.
No, "tomb" is a singular common noun, not a possessive noun.
The singular noun is tooth.The singular possessive noun is tooth's.
The singular possessive form for the noun clock is clock's.
The noun vest is the singular form.The noun vest's is the singular possessive form.
No, "she's" stands for she is. The singular possessive noun for a female is her or hers.
The singular possessive noun of grandmother is grandmother's.
The singular possessive noun for nature is "nature's."
The singular possessive noun for lioness is lioness' or lioness's, whichever you think sounds better.
The possessive form of the singular noun son is son's.Example: Your son's grades have improved this semester.