Did your father-in-law have a trust and/or will?
As others have stated, the executer's duties are to execute the wishes as stated.
If there is no trust and/or will, probate kicks in and the lawyers win.
It sounds like the estate is not that much, so probate is probably mute.
Regarding the cars, generally a trust states all assets not specifically listed in the trust become part of the will.
Perhaps get the family together and discuss options.
If he is not willing to co-operate, contact the authorities.
Unfortunately, there might not be much they can do.
But move quickly before there is nothing left.
Good luck.
FYI, I felt so much better after my trust, etc were in place.