Ben or Bas Nidah – marry

Igros Moshe (E.H. 4:14) Avoiding marrying a woman over concerns that she is a bas Nida as her parents are not observant Jews, even though she is modest and a good person is G-d fearing and loves Torah,. The Rambam (Issurei biah 15) and Shulchan Aruch (E.H. 4:13) state that the child of a Nida is blemished but is not a mamzer. All the commentaries state that the child is fit to marry even a cohen but the problem is he is blemished and therefore best to avoid. Therefore the question is whether to avoid marrying them because of this blemish? In particular this woman even though she is modest and a good person and refined and apparently has overcome the spiritual damage - that perhaps her children  will be bad. However it is also possible that the mother was not actually a Nida because she might have gone swimming in a lake - and even a bathing suit does not prevent her from becoming purified. Furthermore our Sages (Kallah) Rambam and Shulchan Aruch described the blemish of ben Nida as being bad personality traits such as brazenness. And even though we have authorities that a mamzer is a brazen person, we don’t rely on that to say that if the person is not brazen  he can’t be a mamzer The Nachlas Yakov said that this just means if a person is brazen he should be suspected of being a mamzer and needs investigation. However since a ben Nida is not prohibited to marry either by the Torah or rabbis it is possible to rely on the existence or absence of bad personality traits to determine whether she is a bas nida, and in this case with her good midos to assume she is not a bas Nida and that the mother must have gone swimming in a body of water similar to a mikve. Similarly the son of sinners who is modest and refined can be assumed not to be a ben Nida. 

Rav Sternbuch (5:313): Should a Ben Torah marry a bas Nida? Regarding the status of bnei Nida there is a dispute between the Taz and the Shach in Shulchan Aruch (Y,D.196:12). The Taz views it has a problem of yichus (pedigree) and is the result of a prohibited relationship while the Shach views that it causes the child to have a  bad personality because of the spiritual impurity. I have already cited the view of the Chazon Ish said that Torah and mitzvos have the power to purify a person from this blemish and thus rise higher than a person borne in kedusha and is not a ben Nida. Thus he and other major authorities actually ruled not like the Taz… It is possible to determine whether she has good midos and if so he should act quickly and not delay. So even though yichus is very important, it is not a reason to delay marriage. After getting engaged you should have minimal meetings with her prior to the wedding. You noted in your letter that a local rabbi advised you not to marry her for the sake of you families honor. In my opinion this is not a concern for the future, rather you should not delay for this reason because delaying might cause sin and damage your spirituality