Can a widow marry her husband's sister's son?

There is no legal reason why you can't, though you would need to show proof of her death via death certificate before you could actually marry. The only problem would be other peoples' reactions when they find that you & your late sisters husband are getting married. As long as you are both ready to deal with whatever negativity others throw at you because of this, thee's no reason you can not. You should, however, give it some real thought & be absolutely certain it's something you want to do & make sure you are not doing it for the wrong reasons.