She definitely needs a doctor, if she was unprotected this could be an STD (assuming she is not in a monogamous relationship based on the infrequency of the "encounters")
Also, if she was protected by a latex condom, she probably is having a reaction to it...Basically an allergy and needs some benadryl and cream. Good luck to your sister.
(((Raynbow
By the way, there are condoms that are made out of things other than latex if this is a latex allergy. She can use the other kind, just don't use lambskin it's not as safe. Also, the word for the surrounding folds of skin around the vagina is Labia.