I found out 5 years ago that my dad has fathered another child who is female and younger than me I was born 1981 I don't want to upset my mum or dad but I want to know about our other sister but don't no where to start

Do you have her name?

The only way to know 100% is to ask either of your parents for more info.

It is impossible to find her with the little info you posted.