"This relationship is not a moment of madness. I would never be together with someone I wasn't in love with," the 26-year-old Barbara told Vanity Fair magazine's Italian edition.
"Alexandre is a person it's wonderful to share things with," said Barbara, who is a board member of Italy's Serie A team AC Milan, which her father owns and Pato plays for.
Barbara and Pato's relationship is believed to have begun around the time Barbara became an AC Milan board member, but she said she had no qualms about it.
"I feel like a normal young woman who has fallen for a man who attracts her. I don't consider it a moral issue or one of status," she said.
Both Barbara and Pato have been married. Barbara has two sons - Alessandro and Eduardo - with estranged husband Giorgio Valaguzza.
The separation from her husband at the start of the year was "painful for all concerned", Barbara said, but claimed she has grown apart from Valguzza, who she had been with since she was 16 and he was 22.
"I think that over time our characters and aspirations developed, and ten years on, we found we were more mature but more distant," Barbara said.
She, however, declined to comment on the alimony problems Pato has been reportedly having with his ex-wife.
"I'd prefer not to talk about that. Our romantic pasts are important to both of us, but we are just taking things one day at a time," she said.
Barbara is the oldest of three children Berlusconi had with his second wife Veronica Lario. She is a board member of her billionaire media magnate father's Fininvest company.