Hi thanks for your question.
Streptococcal pharyngitis(sore throat
) is contigious and you might have got pharyngitis from your sister.
Most probably thermometer could be a source of infection.Irritation and mucus at back of throat is part of pharyngitis.
I would suggest you to get yourself examined by physician.
You may need a course of antibiotics,antihistiminics and nasal decongestant.
Hope this answers your question