Israeli heart- You wrote that it is "FORBIDDEN". I would like a source for that. Yes Halitza is to performed in a situation where the brother (of the man who did not have children) refuses to marry her. BUT in the case that she does have a child and the father dies I don't see why it would be FORBIDDEN for her to marry his brother. I wouldn't think it would be normal yet not forbidden and the children would most definitely not be mamzerim.
Leviticus 18:16.
Also explained here:
http://www.chabad.org/library/article_cdo/aid/468337/jewish/Prohibited-Marriages.htm(e) His sister, half-sister, his full or half-brother’s wife (divorced or widowed) except for Levirate marriage with the widow of a childless brother. and the full or half-sister of his divorced wife in her lifetime.
And, as I noted in my previous message...Yibbum leads to the uncomfortable feeling of being dangerously close to incest because even though the man would be obliged to marry his sister-in-law out of duty, to be attracted to her and want to be with her as a man would desire a woman, it would be LIKE incest. So the Chalizah is preferred.
There are cases where a Yibbum is not required. Let's say a man and a woman were married for 50 years and never had kids. Now the widow is 95 years old, and her dead husband's brother is 97. Would there be a Yibbum required? NO. Because she is no longer able to bear children and therefore Yibbum does not apply. Likewise if a woman who can bear children becomes a widow but her brother-in-law had an accident and is known to not be able to sire children, then a Yibbum is not required either (but a chalitza is preformed according to some rulings). If a man died childless, his wife is not required to marry her maternal half-brother, because the brothers do not share the same father. And thus Yibbum is not required.
If a man had Jewish children from a previous relation and then marries a woman but then died while she never had kids, she would not be required to marry her brother-in-law because her dead husband already had children and thus Yibbum would not apply. Furthermore under Leviticus 18:16- she is NOT permitted to marry her brother-in-law anyways.
Please note, that the article from Chabad also talks about how it is prohibited to marry a sister-in-law IF the wife is alive or divorced, as we have discussed in the thread.
I'm not just making it up, I have studied and these are my conclusions based of studies and lectures, just have to gather my sources. In any events, most of my sources for this also are found in the Mishneh Torah, apart from knowing the prohibition in Leviticus. But it would be hard to pin point the exact location of where I get my information. Also, I am uncertain if a brother-in-law and the widow sharing the status of sh'niyot under Rabbinical prohibition carries the same consequences under gillui arayot such as mamzerut would. But searching about it online, I found this as well:
http://www.jewishvirtuallibrary.org/jsource/judaica/ejud_0002_0013_0_13339.htmlmarriage with his brother's widow (except in the case of the levirate widow) or divorced wife: such marriages are punishable by karet (Yad, Issurei Bi'ah, 2:1, 9; Sh. Ar., EH 15:22, 26; 44:6; see also *Levirate Marriage)
Furthermore, in Mishne Torah Chapter 1 Halacha 22:
"If she gives birth [different rules apply]: If the child dies, even on the day it was born, the yavam should divorce her, [give her] a get and perform chalitzah with her. 57 [Only] then, is she permitted to marry another man. If the child lives for 30 days after its birth,58 [the child] is considered to be viable, and there is no need for a divorce, for [relations between the two are forbidden by] a severe prohibition].59
If anything we see that in case of a Yibbum, chalitza and divorce under some situations are extremely prefered, as it says "relations between the two are forbidden by a severe prohibition"
I have also found this as well:
http://www.chabad.org/library/article_cdo/aid/558049/jewish/Yibum-and-Chalitzah.htmSexual relations between a brother and sister-in-law are forbidden by the Torah unless within the context of the mitzvah of Yibum. For example, if the widow's husband did not die childless, it would be forbidden for them to marry. There is an opinion expressed in the Talmud that if the Levirate couple are intimate with each other and their intention is not to fulfill the mitzvah, rather they are attracted to each other's beauty or personality, then they have transgressed the prohibition against forbidden incestuous relations!