I have a car that I am no longer able to make the monthly payments on. My brother in law wants to take over is it legal WA ST

While letting a third party (such as your brother-in-law) take over payments and use of the car may not be in compliance with a borrower's agreement with his lender, some people do let third parties assume the loan (without letting the banks know). As long as the payments are paid in full and on time and proper insurance is kept, most banks likely do not go around checking who are actually driving the cars.

Problems often arise because things do not always go smoothly. For example, the third party would take the car, make a few payments, then stop making the payment while keeping the car. Or, the third party crashes the car into other vehicles resulting in huge losses.

You can review your loan documents to see what must be done if you want to assign the loan to some other person.