I think witnesses would say (in typical JW circular logic) that "anything is possible for Jehovah", so therefore he could sin, but he chooses not to.
In line with the same idea on whether God knew in advance whether Adam and Eve would sin - he could have known, according to JWs, but chose not to look into the future to see.
Of course, the usual definition of "sin" is doing something against the will of God, so the question becomes (in that case) - can God do something against his own will. Thus being an essentially unanswerable hypothesis.
And, of course, the silly witness statement that God "could have looked ahead and seen whether Adam would sin" is tantamount to saying that Adam was predestined to sin (i.e. - he had no "free will") - a Calvinist predestination tinged idea that witnesses themselves denounce.
So this kind of foolish head games would just as well ask the classic: "Can God make a rock so big that he cannot pick it up?"