I do not believe the estranged husband will be able to sell the car since it is not in his name. At least if he does, the buyer will be in for a surprise when he goes to transfer ownership and license it.
Who did your sister leave her estate to? Who did she leave the car to? Does the car have liens against it? If the car had a loan or lease against it, it will need to be paid off first or it will belong to the person holding the loan.
Whose possession is the car physically in now?
Are your sister's assets frozen? Has the will been probated?
The new owner of the car (whoever it passed to when she died) will need to take the legal paperwork showing proof they are the new owner to the transportation bureau to prove they're the owner and get it transferred into their own name.