The car must be only used to carry the disabled person. Not for the benefit of others. Not true.
I'm pretty sure that Motability changed the rules a few years back which said that except in exceptional circumstances the Motability vehicle has to be kept at the address of the disabled person. It was precisely to stop this type of 'fraud'. So they've presumably lied to Motability . Also Not True
I have read rules carefully and I can only drive car if foster child is in car or if coming back from dropping him off or going to collect him and the rest of the time I have to drive my own car when foster child is not in car and I'm not doing something for him. Your sister can only drive the car when your Mum is in it or she is collecting your Mum and your Mum's car should be kept at your Mum's address. I'd be reporting sister if she abused the motability scheme which is meant to support disabled people not their relatives. Also Not True
I have a Motability car and I can tell you that if she gets stopped and she cannot PROVE that she is doing something for your mum's benefit then she will be deemed as an uninsured driver and will be prosecuted and so will your mum. The car will also be seized. This is a very serious issue and you MUST report it! Also Not True
There are so many things on this thread which are complete nonsense. The mobility vehicle is paid for by the claimant using funds they would otherwise get in cash. It is not the property of Motability and it is certainly not tracked.
The only rule about having one is that it must be of benefit to the person paying for it. That could mean a parent going to work, a daughter doing parents shopping, the whole family going on holiday and a million other things which mean the person paying for the car has some added benefit to their lives even if they are not actually in the car. Only 2 people can be insured on the car (with the exception of multiple carers) and, in most cases, the insured person (if not the claimant) must be over the age of 25. Using a motability vehicle does not mean that the other drivers can use blue badge spaces (unless the claimant is with them).
People,speak about motability cars as if they are a free resource. They are not. Most cost all of the 260 a month that you would otherwise get in cash if you chose not to get the car.
OPs relative is certainly using the car fraudulently as evidenced by her paying her mum the amount mum has deducted from her payments to pay for the car. If she has the car she does the lions share of the work that benefits her mum.