Beyoncé and Jay-Z worried their oldest daughter would ''feel left out'' when they had twins.

The couple welcomed Sir and Rumi into the world in June and were relieved that five-year-old Blue Ivy quickly adapted to no longer being an only child and has proven to be an ''amazing'' older sister.

A source said: ''Beyoncé has been very busy with the newborns, but she's also been able to relax a bit. It's helped her that Blue has been such an amazing big sister.

''There was no need to worry because Blue is great. She takes her job as a big sister very seriously and helps out a lot.''

Before the babies arrived, the 'Crazy in Love' hitmaker ensured her focus was on Blue and ''made sure to spend lots of quality time'' with the youngster.

The family recently splashed out on an $88 million mansion closer to their oldest child's school and Blue can't wait to see her friends in kindergarten in the next few weeks.

The insider said: ''Blue is excited about starting kindergarten. She misses her friends. It will be fun for her to get back to her own life at school. Beyoncé is excited for her too but keeps saying that she can't believe Blue is starting kindergarten. She thinks Blue is growing up way too fast!''

It was recently revealed Beyonce, 35, and her husband - who are worth more than $1 billion between them - ave a $52.8 million mortgage on their new Bel Air home.

According to the Los Angeles Times, if the pair's mortgage had an interest rate of four per cent they would be paying more than $252,000 per month for the plush pad over the next 30 years.

If the couple ever need their own space they should find some in the 30,000 square feet of living space, or they might like to treat themselves to a relaxing session in their spa after a tough day recording songs.

Jay, 47, is well known to be a big fan of the Brooklyn Nets basketball team, and he has enough space to invite them to Los Angeles for a game as his house boasts a full-size court.

There is also a media room which could be ideal for promoting their latest ventures to help pay for their mortgage, as well as separate staff quarters, four outdoor swimming pools and a wellness centre.