Charles Ellicott Commentary 2 Samuel 13:2

"And Amnon was so vexed that he fell sick because of his sister Tamar; for she was a virgin; and it seemed hard to Amnon to do anything unto her." — 2 Samuel 13:2 (ASV)

Thought it hard. —Rather, it seemed impossible to Amnon. The modest seclusion of Tamar in the harem of her mother seemed to leave him no opportunity to carry out his desires.

It appears from the narrative that the king’s children lived in different households, and each grown-up son lived in his own house.