In MT, Hilkhot Yibbum veChalitzah 7:11, the Rambam presents the case of three brothers, two of whom are married to two sisters and the third of whom is married to an unrelated woman. In the event that one of the brothers who is married to one of the sisters dies, the childless widow is forbidden to the brother who is married to her sister, but may perform either chalitzah or yibbum with the third brother. Supposing that the third brother marries her, that her sister (who is married to the first brother) subsequently dies, and that he (the third brother, currently married to two unrelated women) dies after her, the twice-widowed woman is forbidden to the remaining brother since she was already forbidden to him once before.
To phrase it differently, imagine that Reuben, Shimon and Levi are brothers, that Reuben and Shimon marry Rachel and Leah - both of whom are sisters - and that Levi marries Miriam, who is related to none of the others. Reuben dies and his wife, Rachel (who is forbidden to Shimon, since he is married to her sister) performs yibbum with Levi. Subsequently, Leah dies, after which Levi dies as well. Levi's widows, Rachel and Miriam, now come before the remaining brother, Shimon, who is forbidden to marry Rachel on the grounds that she was already forbidden to him once before.
This same scenario (almost verbatim) appears in the Mishna (Yevamot 3:7), on which the Rambam comments that no law is provided - neither in the Talmud nor in the writings of any of the geonim - as regards whether or not this remaining brother (Shimon) can perform yibbum with the other widow (Miriam: the one who is unrelated to the two sisters). He rules strictly, that she should perform chalitzah just in case, but famously changes his mind between writing his commentary on the Mishna and writing his Mishne Torah, and rules in the later source that she is exempt from both chalitzah and yibbum.
In trying to understand the Rambam's reasoning for this new opinion, I am struck by another difference between the two passages: in Hilkhot Yibbum veChalitzah 7:11, the Rambam also declares that the first of the two women (Rachel: the twice-widowed woman, whose sister was Leah) is forbidden to the remaining brother (Shimon), not only because she was already forbidden to him at one time (that time being when her husband, Reuben, died), but also in accordance with a seemingly unrelated principle: that a man is forbidden to marry the widow of a brother who died before he was born (כדין אשת אח שלא היה בעולמו).
My question is two-fold:
1) What is the relationship between this law (אשת אח שלא היה בעולמו) and the situation under discussion (נאסרה עליו עולמית משום שנאסרה עליו שעה אחת)? How are they in any way similar?
2) Does a recognition of the relationship between these two principles shed light on why the Rambam might have felt that her co-wife was also forbidden to him, and thus exempt from needing to perform either chalitzah or yibbum - in contrast to the ruling previously published by him?
The relevant passage in the Rambam reads as follows:
שלשה אחין שנים מהן נשואין שתי אחיות והשלישי נשוי נכרית. מת אחד מבעלי אחיות ויבם הנשוי נכרית את אשתו ואחר כך מתה אשתו של שני ואחר כך מת השלישי ונפלו שתי נשיו לפני השני שאין לו אשה הרי אלו פטורות מן החליצה ומן היבום. האחת מפני שהיתה אחות אשתו בשעה שמת אחיו הראשון נאסרה עליו לעולם משום אשת אח כדין אשת אחיו שלא היה בעולמו והנכרית משום צרתה.
The part that I have difficulty with is in bold.