Presque personne ne pense que nous sommes sœurs.

English Translation

Almost no one thinks that we are sisters.

Should this be subjunctive? (nous soyons instead of nous sommes) negative of penser is followed by a subjunctive, I thought. Is this an exception because of “personne” or
for another reason?

I agree it should be soyons, otherwise it would mean that nobody is currently thinking about the fact that they are sisters