Jenifer's father-in-law is my brother. Helen is a grand-daughter of my mother. How is Helen related to…

Jenifer's father-in-law is the speaker's brother, so Jenifer is married to that brother's son. Helen, a granddaughter of the speaker's mother, is a daughter of the speaker or of one of the speaker's siblings and so belongs to the same generation as Jenifer's husband; read as the brother's daughter, she is the sister of Jenifer's husband, which makes her Jenifer's sister-in-law. Mother and daughter-in-law are ruled out by generation, and nothing makes Helen Jenifer's own sister.