The accused has been arrested, they said.
The 28-year-old confessed to raping the baby on under the influence of alcohol, said a senior police officer.
The girl's parents used to go out for work and leave their daughter in the custody of their sister-in-law. Since it was a Sunday, their sister-in-law's son was at home, he said.
When he saw that his mother was not around, the accused allegedly forced himself on the baby, police said.
When the girl's mother, who works as a maid, returned home at around 12.30 pm, she saw blood stains on her daughter's clothes and informed her husband.
The baby was rushed to a hospital where it was found that she had been sexually assaulted, police said.
Subsequently, the police were informed and a case was registered.
The baby underwent a surgery today and her condition is stable, they added.