Because the relative can refuse to accept the vehicle or refuse to allow the tow truck on the premises.
It’s also an expense for the OP. Far better to pay £250 for a solicitor to write a letter saying move your vehicle or there will be consequences, and then pay for the vehicle to be sold/delivered, or even incur no expense if the relative moves it. Better that than pay hundreds for its safe removal, only to be forced to accept it back and be sued for damaging it.
The OP has not updated the thread, probably on legal advice.
I expect we will either never know, or she will return in 28 days (minus however many days ago she instructed solicitors) to say I did this, the relative did this and then I did this.
It is really easy to advise the OP to just go ahead and take a particular action, when it’s not your money, reputation or family relationship on the line,
She has explained why she wanted advice. She has consulted a solicitor. We can only hope we eventually get to find out what happened. Personally I can live without that as long as the OP gets the car safely and legally off her property.