Because of the timeframe of when your sister and you were born, it is quite unlikely that your sister and you are British citizens. The chances are ever so slightly higher for you, but they are practically non-existent for your sister.
The document you would want to reference is Table A of the
Treaty Rights Passport Applications guidance.
As you were born before 2nd October 2000, if you have documentary proof that your father was exercising treaty rights (such as by working in the UK) on the date of your birth, you may be able to apply directly for a British passport. My understanding of the guidance listed above is that the length of time that your father was working or exercising Treaty Rights in the UK is immaterial.
For your sister, she would only have acquired British citizenship at birth if either one of the parents would have had an Indefinite Leave to Remain stamp in one of their passports pre-dating her birth. At the time, ILR stamps were issued on demand to EU citizens. But given that it was not a requirement, few EU citizens demanded them. So you would need to do a thorough search through all the passports of your parents pre-dating your sister's birth.
If you find an ILR stamp in either of your parents' passports pre-dating your own birth, then both your sister and you can claim British citizenship.
However, in any case, as both your sister and you were born in the UK and have lived here for the first 10 years of your life, if you do not find the proof listed above, both of you have a lifetime entitlement to register as British citizens on
Form T. Be aware that if you go down this route, any citizenship acquired is only prospective from the date of the registration certificate and will not impact any children born to your sister or you before the date of the registration certificate.