Pointing to Riya, father of Ritu says, she is the daughter of the daughter of the wife of the only son of the grandfather of my sister. How is Usha related to Riya if Usha is the sister of Ritu?
Ritu’s father’s sister’s grandfather is Ritu’s father’s grandfather. His only son can either be Ritu’s father’s father or Ritu’s father’s uncle. So, his wife is either Ritu’s grandmother or her grand aunt. Her daughter would be Ritu’s father’s cousin or his sister. So, Riya is either Ritu’s aunt’s daughter (cousin) or her father’s cousin’s daughter (second cousin).
Hence: Usha is either sister to Riya ( sibling) or cousin sister. Hence either sister or cousin is the correct answer.
NOTE - Cousins share a grandparent. Second cousins share a great-grandparent.