Please understand, first of all, that we here at enotes are not medical professionals. That being said, if I were in your shoes I think I would take her to the doctor. There is very little likelihood of rabies, since that disease is transmitted by the saliva in bite wounds and not by scratches, but the fact that the injury is turning red could possibly indicate that an infection is brewing. A dog's nails are a great breeding ground for bacteria because they are in contact with the ground and everything that is on it, plus the hollow behind the nail can hold materials and dirt. It would be better to see the doctor and be safe than to let an infection get out of hand.
Since i had not seen that dog, if you find any type of symptoms of rabies on the dog than their are many possibilities that you sister might got that diseses. In this type of case you sister may counter with hydrophobia in which you will find that she will afraid of water after the bite. So, if you see such symptoms you must took your sister to him for further treatment.
yes u should take her to the hospital.any cut no matter how deep can be something serious.it may turn into an infection.take no chances.