Are you saying nobody lived at that address for the last X days or weeks and therefore the entire service was not used? Or are you saying someone was there but your sister was unable to watch TV due to her disability?
Here are two scenarios:
Scenario 1: More than one person occupancy
If someone else is living at the property then if you have a TV, PC, Laptop, smartphone or tablet or any other device that is capable of receiving either live or recorded (including catchup) BBC broadcasting whether used or not you are required, by law, to have a colour TV License.
As of August 1st 2020, TV licences were no longer free to the over 75s unless anyone in the household were receiving Pension Credit. This does not have to be the same person.
Scenario 2: Single person occupancy
If someone lives in a dwelling by themselves but then has to move for any reason, they can request a hold on their service for a period of time.
However, if the person is still living at the dwelling but is just unable to use said service then it is, unfortunately, their responsibility (or that of a relative to act on their behalf, a Limited or Full Power of Attorney) to ensure services are either suspended or terminated at cost according to the contract.
Scenario 2 sounds more like your sister's issue here but I would say getting a TV Licence refund, or partial refund, seems to me to be the least of your concerns, I would be more interested in getting my services with Virgin, Sky or whoever at a priority as this will cost you far more than a TV License refund.
Just a word of advice, pay your TV License monthly by Direct Debit. It's less hassle but costs no more than buying quarterly, 6 monthly or annually. If you can't afford to do it m9nthly then set up a payment plan for biweekly payments, again, at no extra cost.
These are only my views; I've given them to try and assist but if anyone finds fault with my posting then please let me know.
Please excuse misspellings and anything which may not make sense or offend as the medication I take can sometimes have an adverse effect on me.
Chris (VMO2 Customer)
Ultimate Volt: ACTIVE
TV: Maxit TV
Broadband: Volt Gig1 Fibre Broadband
Phone: Talk More Anytime
Mobile: O2 Unlimited