Of course he's a son of Ol' Blue Eyes. Look at the incredible resemblance:
I'll bet Nancy Jr. is tickled to death at the thought her father cheated on her stepmother Barbara. There is EPIC bad blood between the Sinatra children and his widow Barbara.
I read both Tina's memoir and Barbara's autobiography. Tina painted her as the golddigging stepmother from hell, while Barbara basically ignored mentioning the children at all except a time or two in a fleeting reference about Frank's "family."
The truth is probably somewhere in the middle.
Getting back to Mia Farrow, she says Sinatra was the love of her life.
I do know his children liked her quite a bit despite the age difference between her and their dad.