Ruling on marrying a woman after proposing to her while her brother had already proposed

Question:
If he proposed to a woman after his brother had proposed to her, and she was married to him, what is the status of the contract?

Answer:
It appears that the contract is valid, but the proposer commits a sin; because the prohibition is against the proposal, not the marriage.
Q: Why was the mahr not announced?
The Shaykh: The mahr comes later, and if the mahr was not specified, she is entitled to the mahr of her like.