Can pire be used in the feminine form, e.g. Marie est la pire?

Hi James, 

Yes you could say 'la pire' in the case of feminine nouns.

C'est la pire des choses = It's the worst of things

C'est la pire erreur jamais faite = It's the worst mistake ever made

C'est la pire décision de ma vie = It's the worst decision of my life

Hope this helps!