Mark 12:20

There were seven brethren: and the first took a wife, and dying left no seed; and the second took her, and died, leaving no seed behind him; and the third like wise: and the seven left no seed. Last of all the woman also died. In the resurrection whose wife shalt she be of them? for the seven had her to wife.

Just why the Sadducees thought this was any greater problem than would have resulted from only two brothers having had the same wife is not clear. The whole allegation of such a situation bears a mark of contrivance and unreality upon it. It was only the Sadducees' manner of making fun of the idea of resurrection. That it was technically possible for such a thing to have happened was true; and Jesus proceeded to answer it without regard to the unlikelihood of any such thing ever having happened. (See exegesis on parallel account in Matthew in my Commentary on Matthew, Matthew 22:24-28).