Respuesta :
Answer:
her sister-in-law's
Explanation:
The possessive cannot be 'sister's-in-law'; it has to be "sister-in-law's", and that is what native speakers say and write.
e.g. The house of her sister-in-law is her sister-in-law's house.
It's her sister-in-law's.
If we had to say the possessive form of passer-by it won't be passer-bys, but passers-by
If we had to say the possessive form of passer-by it won't be passer-bys, but passers-by