I used to have the problem all the the time until I got rid of the post mortum hack. You will have to unlink her in SimPE to get her to remarry.
That doesn't seem to be the direct cause, given that I specifically tested this case and it worked, but it does seem to expose the issue more prevalently and serve as a convenient scapegoat. I'm not sure why sometimes the relational bits that impede remarriages are never removed, but there seems to be no consistency to it. I'll try adding code to force their removal while not disconnecting lines.
UPDATE: New version. Try now?