I just found out that someone I know will be interviewing me for a position. It is my brother's good friend's sister - so someone I am acquainted with but don't know too well. Should I let the company know? They would be my superior if I got the job so would this jeopardise my chances of getting it?
When you work for a company and are applying for an internal job change or promotion the HR person, the hiring manager, and people doing the interview might all know you.
When you are trying to find a job, leveraging who you know to get an interview it is a important job search strategy.
In your case they would be responsible for letting people know that they have meet you socially. If they even remember you. They might not. their opinion about you could neutral, or it could be good, or bad. there is no way we can know.
Sometimes these relationships aren't discovered to the day of the interview. Rarely would I told the names of all the people who would be interviewing me before I arrived. I might be given one or two names, but that would be it.
I have also not been given the resumes of the candidates until the night before an interview, or just a few hours before I was to call a candidate as part of a phone screen.
I wouldn't worry about it. If you are concerned you might be able to bring it up during the introductions. You could say, "by the way we have met, our brothers are friends."