Britney Spears has written “some really silly” songs with her sister.

The singer has been promoting her eighth studio album Britney Jean of late, which includes some tracks with her sibling Jamie Lynn, 22.

It’s not the first time the pair have worked together, although usually it’s a little more relaxed than a full on studio session.

“Whether it's listening to some of her tracks or it's me playing the guitar or playing the piano or just us messing around as sisters, we've written some really stupid, silly songs,” Jamie Lynn told MTV News.

“It's really just so fun when you have someone who appreciates the writing process and you're so comfortable with. We've never finished anything... but we do start."

The pair’s new song is called Chillin’ With You and it was Britney’s idea for it to feature on the record. Jamie Lynn’s daughter Maddie, five, helped out by dancing in the studio and both stars are excited to hear what fans make of it.

“She was like, 'Why don't we try singing this together?' and I got in there and I literally sang it four or five times and it turned out really cool and we loved it," Jamie Lynn explained. "As sisters it's just fun to do something like that - it was just a really natural, organic thing.”

Jamie Lynn is also a singer, although she is now in the country arena rather than releasing pop tracks like her sister. Britney has already said she’d like to dip her toe in her sister’s genre of music, which is something Jamie Lynn would love to happen.

“I think it would be cool for her to switch it up, and obviously just for me to do another song with my sister, I would be totally open to that,” she gushed.

Britney’s two-year Las Vegas residency kicks off tonight.