Commentary for Yevamot 10:1

One who betrothed a woman and (she) went to a different country and witnesses testified that she died and he went and married her sister and after the fact (it) was revealed that she had not died - he is forbidden to the both of them and the both of them require a divorce from him, as well, all of the other matters listed in the chapter HaIsha Rabbah (lit. The Great Woman, one of the few chapters in Mishna which begins with the word HaIsha, the woman, the word rabbah, great, a discription about the chapter which differenciates it from others with a similar name, Yevamot 87b) and chapter HaZorek (lit. he who throws, Gittin 79b) apply to them. If one of them died, he is permitted to marry her friend (her sister). However if his wife (fully married via nesuin and not merely betrothed) traveled to a different country and witnesses came and testified that she died and he married her sister and then it was understood that his wife did not die - her sister does not require a divorce from him and his (first) wife is permitted to him, and he is permitted to marry the relatives of the second (wife) and she is permitted to marry his relatives and if the first dies he is permitted to the second one. And so (applies) to all 'Arayot (certain forbidden relationships) that one married under the chazaka (halachikally legitimate status-quo assumption) that they were permitted and were then found to be 'Arayot- they do not require a divorce from him because kiddushin (halachikally binding betrothal) does not operate (lit. catch) with 'Arayot. So too all acts harlotry one committed with relatives who are forbidden to him because of his wife do not forbid his wife to him and they (the forbidden relationships) do not become forbidden to him (Tur). However, if that relative would normally come to him because of his wife, (the court) forces him to divorce his wife (al-Fasi there). And why did (the sages) require (the giving of) a divorce to the sister of one's betrothed? Lest (people) say "there was a clause in the engagement and he married her sister legitimately," and once she has exited (the betrothal) with a divorce, her sister, his first betrothed, is forbidden so that (people) do not say "he has married the sister of his divorcée." However when his wife (via nisuin) traveled (to another country), there is no need to suspect that (people) will say "there was a clause in the marriage," because one does not (regularly) render his (legitimate) sexual relations to be(come) promiscuous relations; therefore, all will understand that marriage of the second (wife) was meaningless.

They told him: "your wife died," and he then married her sister and later they told him: "(the first time we had told you that she was dead) she was (actually) alive and (only after he had married her sister) died," the first child (conceived before she had died) is a bastard and the last one (conceived after the wife's death) is not a bastard.