Ross went to his father's sister's daughter's maternal grandmother's only…

Concept: To solve a multi-step blood-relation chain, resolve one relationship at a time, converting each descriptor into a concrete position in the family tree before moving to the next descriptor. Two rules used here: (1) a person's maternal grandmother is their mother's mother, and (2) a person's son-in-law is their daughter's husband. A child of one's parent's sibling is one's cousin.

Application:

1. Ross's father's sister is Ross's paternal aunt.

2. The aunt's daughter is a cousin of Ross on his father's side.

3. That daughter's maternal grandmother is the aunt's own mother — which is also Ross's father's mother, i.e., Ross's paternal grandmother.

4. The paternal grandmother's only son-in-law — the husband of her daughter — is, from the family tree, the aunt's husband.

5. The aunt's husband's son is the aunt's own son.

6. A parent's sibling's child is a cousin, so the aunt's son is Ross's cousin — and it is his house Ross visited.

Cross-check: The family-tree diagram for this item traces the same path independently — labelled nodes for Ross, his father, his aunt, her husband, and her son — and marks the link between Ross and the aunt's son directly as 'Cousin', matching the step-by-step trace above.

Result: So Ross went to his cousin's house.