@Quveas I agree.
I'd be happy for either of my brothers to have mums motability car & for them to be the ones who get called away from work, have a very understanding employer so they can take time off to go to her numerous doctors/hospital appointments, get up & start work at 5am so they can take 3 hours out of their day to take her to a friends funeral (because it's on the busiest work day & tasks are time sensitive), they could go to hers every day before & after work to do her housework, get her up & dressed, make her breakfast, make dinner, prepare a lunch, help her with showering & getting her to bed. They could then give up their weekends to ensure she gets out of the house, get her shopping, take her to the cemetery, visit friends/relatives, do her Christmas/birthday shopping, deliver the Christmas/birthday gifts to the multiple grandchildren & great grandchildren she hasn't seen all year.
And while I would still be doing all of those things without the motability car having it makes those things a lot easier for both me & mum.
So yeah if I take the car to meet a friend for coffee on a Saturday morning before heading to mums I don't feel guilty that she's not in the car or that I'm doing something for me - it's still for her benefit because if I didn't get that little bit of respite I'd probably have a breakdown.
And motability obviously don't care as when they check the tracker they can clearly see my Saturday has been:
Costa
Mums
Marks & Soencers
Asda
Mums
Cemetery
Auntie M's
Mums
Home
OP if you genuinely think your sister is using the car fraudulently then contact Motability & report it.