Responsa על שבועות 93:7

Q. A was survived by his wife, a married daughter, and a single daughter. Subsequently the married daughter died and then her death was followed by that of her mother. The latter had not taken the required oath regarding her ketubah. The unmarried daughter took possession of A's entire estate; but, A's son-in-law demanded half of the estate on the grounds that his wife had been entitled to it, and that he was her rightful heir.
A. As long as A's widow did not take an oath regarding her ketubah, A's entire estate belonged to his two daughters. A's son-in-law, being his wife's rightful heir, is, therefore, entitled to half of A's estate.
This Responsum was addressed to Rabbi Menahem.
SOURCES: L. 226; Mord. Sheb. 780. Cf. Israel Bruno, Responsa 21.

Q. A, his four daughters, and his wife, were killed when the house they occupied collapsed. A's heirs claim that perhaps the wife died first and that therefore they now were the only heirs to A's estate. The wife's brothers, on the other hand, claim that perhaps the wife died last and that therefore they were entitled to her ketubah. Some Rabbis are of the opinion that the wife's heirs were entitled to one-twelfth of the estate; while other Rabbis, computing probabilities, believe that they were entitled to one thirty-second of the estate.
A. It is my humble opinion that the wife's heirs are entitled to one-half of the estate, because of the talmudic principle Kol Kavua kemahaze al mahaze domi, meaning: whenever an event is about to befall (or has befallen) one of a number of fixed persons or objects, each person or object has a fifty-fifty chance that the incident will befall (or has befallen) him or it. Therefore, legally, there is a fifty-fifty chance for each one of the six persons involved that he or she died last. Had the daughters been married and had their husbands appeared to demand their share, the estate would have had to be divided into six parts. But, since only two persons demand their share, the estate should be divided between the two. Although a woman has to take an oath before she is entitled to collect her ketubah, no such oath is required in this case since A died suddenly and we have no reason to suspect that before his death he had deposited with her valuables to be used in payment of her ketubah.
SOURCES: Cr. 172; L. 378; Mord. B. B. 638; Tesh. Maim. to Mishpatim, 5; Agudah B. B. 213; Asher, Responsa 84, 3. Cf. ibid. 85, 1; ibid. 86, 1; Maharil, Responsa 63; ibid. 169; Moses Minz, Responsa 96; Terumat Hadeshen 330.