UK citizenship from a British mother

[Marmaduke]

Hi, I'm helping my sister to apply for UK citizenship, and I keep finding contradictory advice in my online searches.

The facts:
My sister was born in Australia, after 1983, and is over 18.
Her mother is born in the UK, but is now a dual citizen with Australia (still has British nationality). Her father is an Australian citizen.
They were never married, but this consideration only really affects children born to unwed non-British mothers and British fathers, right?

So my question is: what, if anything, does my sister have to do to 'claim' her UK citizenship? I had to use form UK-M to apply for mine (I was born before 1983), but some of the advice seems to be that my sister is already a UK citizen by default. However, say she travelled to the UK on her Australian passport and wanted to access the NHS, how would she then prove that she was a UK citizen?

Sorry if this seems to be a rookie question, and thanks for your help.

[Obie]

Your sister is already a British. Need to prove her mother is British and she will be able to apply for a passport.

[Marmaduke]

Thanks, Obie, I thought this was the case, but there is a lot of bad advice out there.

So she would send in the documents (birth certs of her mother and grandparents) when she applies for the passport the first time, that makes perfect sense, and then the passport would form the basis of building her documented identity in the UK.

Thanks for your prompt help.

[ohara]

I think you should look at Form NS:

https://www.gov.uk/government/uploads/s ... e_2014.pdf

And the guidance:

https://www.gov.uk/government/uploads/s ... update.pdf