The pair sound like typical siblings and favored bickering to getting along until there was a family crisis. "I trace it back to when our mother, was sick with cancer. She's now a 15-year survivor," said Star. "When there was a chance that my mother would not beat that disease, I think Sheila and I somehow thought we had to be together. And we started really working on our relationship."
The sisters became even closer when Sheila had the first of her two boys. Star is kind of an Auntie Mame for all her family these days, but despite her glamorous life and fame, her sister insists she has not "an ounce" of jealousy.
"I would not do what she does for all the money in the world," said Sheila. As it turns out, Star is the one with the envy. "It's funny. People always think Sheila should envy my life … I always envy her life," said Star. "I see the joy that her boys bring her."
Can being a teen heartthrob run in the family?
It does if your last name is Cassidy. When The Partridge Family star David Cassidy gave up his idol crown in the mid-1970s, the teen machine sought to replace him with his half-brother, Shaun Cassidy.
David Cassidy told ABCNEWS he was not surprised Shaun was interested in following his lead. "How could he not be?" said David. "It seems real attractive when your older brother's you know, 20 and you're like 15 … everybody's talking about him and everybody's playing his records and buying his stuff. You want to do that, of course."
So Shaun took a role on the Hardy Boys, scored a handful of hit singles and was marketed as the next Cassidy pop-idol. And with a half-brother in the business when Shaun had career struggles, he had someone to turn to for support.
"We were able to talk a lot about, because I'd already been through years of it," said David. "He realized that he was stuck in this TV show and he had to make these kinds of records, and creatively felt very stifled."
Shaun would later step behind the camera to become a respected TV producer and David is still in music, with a tour planned for this summer. What's their secret to sibling serenity? David said it's a matter of remaining supportive: "Can you find happiness in having a relationship, with your family, with your sibling, can you appreciate the fact that their fame … and their success is a good thing?"
Hillary and Haylie Duff
What happens when the younger sibling becomes the bigger star?
That's the case with the now-famous Hillary Duff and her big sister Haylie who was the first to take acting classes. "I thought it was the dumbest idea at first, like when I was younger, because I was such a tomboy, and, like, when Haylie started taking acting classes, I was like, what is that? That's so stupid, you know?" Hillary Duff told 20/20.
She's certainly changed her mind. Hillary now has hit the teen-queen trifecta: on television with the series Lizzie Maguire, in the theatres with two hit films, and on the charts with her No. 1 CD Metamorphosis. All that success could have been tough on an older sister with showbiz dreams, but for these sisters the success is not a strain.
"We never really thought about it before!" said Haylie.
Hillary is confident her sister will succeed in show business as well and said, "It's just a matter of timing."