“Now there were with us seven brothers, and the first married and deceased, and having no seed left his wife to his brother, in like manner the second also, and the third, to the seventh, and after them all, the woman died.”
They then laid out the case where seven brothers died childless one after the other, each taking on the same wife in order to produce children for their brothers, after which the woman also died. Note the sad emphasis on a hopeless death. There was no resurrection here, not even on a Sadducean interpretation! All died and no life resulted.
The sevenfoldness was probably an exaggeration in order to emphasise the completeness of the argument, but it remained true, of course, if it occurred in cases of fewer brothers (three or more). ‘With us' may indicate that an actual case was known. It would certainly not be impossible. But it was probably said more with the intention of emphasising the veracity of the argument.