Ankit is the brother of Vishal. Kartik is the son of Arpit. Malini is the mother of Oshi, who is the sister of Vishal. Ankit’s father is the only son of Arpit. How is Malini related to Kartik?

Let's break down the statements to construct the family tree:
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Ankit is brother of Vishal → Ankit (Male) and Vishal are siblings.
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Malini is mother of Oshi who is sister of Vishal → Oshi (Female) is a sibling to Ankit and Vishal. Since Malini is Oshi's mother, she is the mother of Ankit, Vishal, and Oshi.

Ankit's father is the only son of Arpit → Since Malini is the mother, Ankit's father is Malini's husband. Let's call him 'Father'. He is the only son of Arpit.

Kartik is the son of Arpit → Arpit's only son is Ankit's father. Therefore, Kartik must be that only son, meaning Kartik is Ankit's father.

Since Kartik is Ankit's father, and Malini is Ankit's mother, Malini is the wife of Kartik.