If your cat is regularly immunised at the veterinarian and the scratch was a light one with some blood you need not worry.
However if your cat has missed any dosage and the wound on your grandmothers hand is deep then it is better you get a vaccine .
Also, you have mentioned that your cat is healthy. There is no need to worry now.
Health Tips
Always wash wounds due to bites or scratches by animals under running water as soon as possible.
Don’t miss the vaccination schedule of your pets.
Incase, of bite by suspected rabid animal or domestic animal keep an eye on any behavioural or health change’s in that animal for atleast 15-20 days. Incase of deterioration , the person bitten/scratched May need medical help.
Rabies is required if the animal is stray/rabid or missed its vaccination with wounds and blood loss.