Can you be approved for a British Passport/Citizenship if you don’t have your parents’ passport number/information?

My mum (born and lives in Australia, holds only an Australian Passport) is wanting to apply for a British passport/citizenship as her father was born in Leeds, England. Her sister (full sibling) was issued a British passport in 1995 but my mother is worried the laws/requirements may have changed since she’s quite sure her sister didn’t have their father’s passport number (they have been estranged from him since their childhood). My mother has a copy of both her own and his birth certificate and her parents marriage certificate. She was born in 1967 and her parents were married at the time of her birth. Is she eligible for a British Passport and citizenship like her sister with the documentation she has? And also, does this make her a British national?
Answers

Brooky

She doesn't need any passport numbers. Apart from her own identification, she needs her father's British birth certificate and her parent's Marriage Certificate. She has those (they must be certified and not simply copies) so that's all she needs to prove her claim to British citizenship and can therefore apply directly for a British passport as she is already automatically a British citizen and has been since birth. As a British citizen 'by descent', she cannot pass her citizenship to you or your siblings unless you were born in the UK. https://www.gov.uk/apply-citizenship-british-parent/born-before-1983

Maxi: https

https://www.gov.uk/apply-citizenship-british-parent

Tavy

You are assuming that everyone in the UK has a passport, well a huge amount of people do not. So it is not compulsory. As for the rest she needs to look it up.

ibu guru

You need proof of your parentage & proof of your parent's citizenship at the time you were born. Passport number alone is not proof, and not required. If your mother gets citizenship by descent, that does nothing for you. Citizenship by descent is only allowed for one generation & cannot be passed on.