Jessica Simpson "burst into tears" upon meeting her new niece.
The 35-year-old star, who has daughter Maxwell, three, and son Ace, two, with husband Eric Johnson, is reportedly desperate to have another baby after falling in love with her younger sister Ashlee Simpson and husband Evan Ross' newborn baby girl, Jagger Snow.
A source close to the fashion mogul said: "[Jessica] burst into tears when she first saw her. She forgot how gorgeous and little they are."
The blonde beauty, who was criticised for the amount of weight she gained during her first pregnancy, previously claimed she didn't want any more children, but has had a change of heart since Ace was born in June 2013.
The insider said: "After finally going through a really good pregnancy second time around, she's not put off by doing it again."
But former NFL star Eric is reluctant to expand their family further and he is said to be considering undergoing a vasectomy.
The source told the U.S. edition of OK! magazine: "Convincing Eric is another story, but she's determined to make this happen."
Jessica admitted earlier this year that she could "accidentally" have another child in the future.
She said: "I say we're done [but] I don't want to accidentally get pregnant one day and then that poor kid feels like it wasn't meant to be or something."