I suggest you sister leaves work, school, home or wherever she is right now and heads to the nearest passport office in person as the passport needs to be valid for at 6 months from the time you leave Bulgaria.
They will not let her into Bulgaria if she tries to travel on her current passport.
Her only option, if she still wants to go on holiday, is to pay the premium same day service where she will have to go to the nearest passport office in person and pay the £120 (about that).