[27b top] Rav: Two brothers (A & B) marry 2 sisters (1 & 2 respectively). When the first two brothers pass (without children), their third brother (C) cannot marry either one of the two sisters (1 or 2) because each carry the prohibition of being “a sister of the one that is now bound to him,” (through the requirement of Yebum). But, once one of the sisters pass away (ie: 1) it is then permissible for the remaining brother (3) to marry the remaining sister (2).
D-E-ep Thoughts: Halacha (Jewish law) must always be analyzed and revisited. It is not an old book that is set in stone. Our laws may never be disregarded, but must evolve to apply to the changing situations and times. We must always seek proper Rabbinic advice and never try to interpret the law ourselves. Even very similar cases may have different rulings based on the people or items involved in the matters. Especially as the circumstances change throughout existence, we must keep our Torah glasses and thinking skullcaps on to navigate through life’s rapids.