Royal Baby Birth: What Prince George's Childhood Will Be Like With His Younger Sister

— -- With the birth of a royal baby girl today, Britain's Prince George of Cambridge is a big brother for the first time.

"Prince George and his little brother or sister will be incredibly close in age, and they will grow up side by side," Daily Mirror royal correspondent and ABC News' royal contributor Victoria Murphy said before the birth. "But of course, there's one huge difference, and that's that George is destined to be king and this baby is not."

Prince George and his baby sister are third and fourth in line to the British throne. That means they likely will have a close bond fostered by sharing something completely unique, as did their father and his younger brother, Prince Harry, before them.

One thing is for sure: William, 32, is determined to give his children a normal life.

"I think that just like Kate, William is very keen for his children to have the upbringing that she had," Murphy said. "She had a very happy childhood. She's very close to both of her parents, who are still together and still very much in love."

"It's a very secure family unit," she said of Duchess Kate's family, which also includes two more Middleton children, Pippa and James. "I think that's what she and William want to give their children."

From Prince George's very first photo op while with his parents on an official trip in New Zealand in April of last year, the Duke and Duchess of Cambridge have tried to ensure their son has some semblance of life as a normal child.

In that first photo op, George was introduced to the world at a playgroup with other children his age. Months later, like Princess Diana did for her sons 30 years ago, Kate brought Prince George to see Santa and he waited in line just like every other child with no special treatment.

"Kate takes him to parks. She takes him to the beach. She takes him to the farm. She wants him to enjoy all of that, like any other child would," said Murphy. "She takes him to carol services at Christmas to mix with other children."

Like many young children, George loves to splash about in the pool and his parents are teaching him to swim, ABC News has learned.

William and Kate currently rely on one nanny, Maria Teresa Turrion Borrallo, to help with George.

Like many other young families, George and his sister will also enjoy the company of their family's beloved dog, Lupo, a wedding gift to William and Kate from Kate's brother James.

"Of course, this baby will be their second child, but in some ways this baby is their third child because you've got to remember they also have Lupo, who was there long before Prince George," Murphy said. "Lupo is their absolute, beloved pet dog, who they treat exactly like one of the family.

"As well as having a great relationship with William and Kate, Lupo and George also get on really well," she said.