Who is my father’s wife’s grandfather’s only child’s son’s daughter to me?

Concept:

Blood-relation puzzles are solved by converting one possessive link of the chain into an actual family member at a time, working from the innermost relation outward. Two facts anchor every step: a parent’s sibling’s child is always a cousin, never a niece or nephew (a niece/nephew can only come from your OWN sibling’s child (never your parent’s sibling’s child)); and ‘X’s only child’ simply fixes which single person carries the chain forward — it does not change how the later links are read.

Applying it to this puzzle:

1. ‘My father’s wife’ is my mother.

2. My mother’s grandfather is my maternal great-grandfather.

3. That great-grandfather’s only child is the parent of my mother — my maternal grandparent (since he has exactly one child, that child must be the one connecting him to my mother).

4. That grandparent’s son is a brother of my mother — my maternal uncle.

5. That uncle’s daughter completes the chain.

Why the other options don’t fit:

• Aunt is your parent’s own sibling, one generation above you; the chain’s last step (‘...’s daughter’) brings the result down to your own generation, not up to your parents’ generation.

• Niece is your OWN sibling’s daughter; every link here runs through your mother’s side, never through a sibling of yours.

• Real is not a recognised kinship term, so it cannot label a person reached through a family-relationship chain.

Conclusion:

Working through every link in order — mother, her grandfather, his only child, that child’s son, and finally the son’s daughter — the person described is your cousin.