Letter of invitation for uk visa?

hi! i want to invite my sister to visit me in UK for 1 week, i sent all my accommodation documents ( council tax bill and utility bills),copy of my passport and invitation letter by mail, but i think the mail has lost , and her interview is in 4 days so no enough time to re send them through the post, i was wondering if anyone advise me if it's acceptable i send them by fax/email? urgent replies please... many thanks