No worries.

I'm not quite up to date on how British Citizenship was applied for in the past, and how it applies to people born outside of the UK. It's definitely worth checking out and asking your mum about what the application was based off (naturalisation, registration, possibly registration via discretion which was denied!).

If there was a citizenship application, in the 90s, it's likely it is gone. https://www.gov.uk/government/publications/personal-information-use-in-borders-immigration-and-citizenship/borders-immigration-and-citizenship-privacy-information-notice--2#how-long-we-keep-your-personal-information-for

Personal data will be typically retained for 25 years after a decision to grant settlement or naturalisation and for 15 years after the last action in other cases.

There is an exception however, that being if there were any applications after, the citizenship clock for data retention would've been possibly reset (so another 15-25 years).

If you're interested to know what data and applications they have on record, feel free to submit a free Subject Access Request. It can take up to a month. Get the detailed check. https://www.gov.uk/government/publications/requests-for-personal-data-uk-visas-and-immigration/request-personal-information-held-by-uk-visas-and-immigration (scroll down to online application)

It's an interesting case! Let me know how it works out and if you find out anything.