Is your sister is having blood pressure ?
If there is sudden rising of blood pressure, many a time there is bleeding from nose or from any part of body.
Go for her blood pressure checked.
If there is congestion in nasal mucosa then also bleeding might be there.
Put some decongestant nasal drops like Otrivin
apply some ice pack on the nose.
If require go for blood checking like bleeding and clotting time, platelet count
Ok and take care