Sia is ''learning how to be a mother''.

The 44-year-old singer announced in May she had adopted two sons, who are both 18 years old, last year and a few weeks later revealed one of them had fathered two children, but she's not planning to talk about them anymore because she's realised they're not as much of an ''open book'' as she is.

Asked about her new responsibilities as a grandmother on 'The Kyle and Jackie O Show' in Australia, she said: ''I've decided to stop talking about it.

''I made a bit of a boo-boo because they're my new children. I've only had them for a year. I'm such an open book that I forget not everyone else is.

''While I didn't get into trouble for blabbing about his [her son's] children - I was peeing, which is where I have my best thoughts, and I realised that it was not my news to share with the world...

''I instantly realised I can't talk about my children's private lives. I'm just learning how to be a mother.''

And the 'Chandelier' hitmaker explained how she's realised what her new ''job'' is.

She said: ''[It's my] job to protect my children and not blab about what's going on in their lives.''

Sia previously joked she was ''immediately horrified'' at the idea of being a grandmother.

She said: ''My youngest son just had two babies. I'm just immediately horrified. No, I'm cool. They call me 'Nana.' I'm trying to get them to call me 'Lovey,' like Kris [Jenner]. I'm like, 'Call me Lovey.'''

The 'Cheap Thrills' singer admitted her sons had been on a ''pretty massive rollercoaster'' in their short lives, including being in ''18 different locations'' in their 18 years of life.

She added: ''They were ageing out of the care system. They have until they're 21, but they were both 18 when I adopted them.

''I'm a little bit jaded now after investigating the foster system as much as I have done in the last year. It's failing us. Not in my experience, in my sons' experience.

''They've been in 18 different locations in their 18 years. Started out as a pretty massive rollercoaster. They told me one thing, I discovered another. But I just stayed really like a kind of Al-Anon Ninja. I just kept really strong boundaries.''