A homeless man kidnaps a baby for ransom. She’s sick. He breaks back into house to steal her medicine. Her parents die before paying ransom. He raises the baby as his own. He becomes wealthy. She grows up thinking she is his daughter then falls in love with her bio brother. The kidnapper takes his own life confessing all in his suicide note. More details below.

The kidnapper was homeless and sleeping on a bench. He had covered himself in newspapers to keep warm. One of the newspapers he was using to cover himself had a story about a kidnapping for ransom.  He got the idea to do the same (kidnap a baby for ransom). So he takes a baby and then hears on the news that she is sick and will die without her medicine. Despite the risk, he breaks back into the house and steals her medicine.

Meanwhile the police find a baby and call the parents to come to the police station to find out if it is their baby. On the way to the police station the parents are in a car accident and die.

The man doesn’t return the baby to her relatives but instead moves to another country and raises her as his daughter. She believes she is his daughter. The man works very hard and becomes fabulously wealthy. The girl grows up and falls in love with a boy. The man is very protective and does a background check on the boy and discovers that he is the girl’s biological brother.

The man cannot bring himself to face the girl, so he kills himself, leaving behind a letter where he tells her the truth.

The book was read pre-1973ish. And to up the difficulty a notch: she would have read it in Spanish, so maybe it was a translation of something English, maybe not.