[Avodah] uncovered hair in home in front of relatives.

[Chana Luntz]

<< Private: The Biur Halachah writes that although originally it was
permitted for married women to uncover their hair in the privacy of their
homes, in more recent times "the prevailing custom in all places is for
women to cover their hair, even in the privacy of their own homes.... Since
our ancestors, in all localities, have adopted this practice, it has taken
on the full force of Jewish law and is obligatory...."
Rabbi Moshe Feinstein disagrees with this ruling and writes that "[covering
hair when in private] is praiseworthy, but not required."
Can anyone tell me where this igros moshe is? >>

See Igeros Moshe Even HaEzer Chelek 1 siman 48 and also (and particularly)
Igeros Moshe Orech Chaim chelek 5 siman 37:12:

כיסוי הראש בפני בעלה, אינה צריכה.  דהאיסור פרועת ראש הוא רק בשוק.   אפילו
בשעת נידותה ליכא שום איסור בביתה בפני בעלה ובניה.   והידור איכא אפילו לעשות
כקמחית (יומא מ"ז ע"א), אבל לא שמענו שאיכא צנועות כאלו ואפילו בדורות הקודמים.
בזמן התנאים לא היו נשותיהן נוהגות כן אלא יחידות כקמחית.
The covering of the head before her husband is not necessary. Since the
prohibition of uncovering the head is only in the marketplace.  And even at
the time of her period, there is no prohibition in her house before her
husband and children.  And there is a hidur to do like Kimchit (Yoma 47a)
but we have not heard that there are any modest like this and even in the
earlier generations.  And in the time of the Tanaim the married women were
not accustomed so except for individuals like Kimchit.

Note specifically *but we have not heard that there are any modest like
this, and even in the earlier generations*.  A reasonably translation of
this is surely: neither Rav Moshe's wife, nor his mother did this.

<<Answer: It is permissible to uncover your hair in your own home in the
presence of your father, husband and son.

Where it is customary and not considered offensive, a woman may uncover her
hair in front of her brother in the privacy of her own home.

Is this leniency known/relied upon? Is this what people are doing out there
today?>>

I think it depends on your community.  In a modern orthodox community in
which most women are not covering their hair when they go out in a public
place either, I suspect many if not most of the few women who do cover their
hair when they go out absolutely rely on this position, and sometimes more
lenient ones inside their homes (ie only cover their hair when they go out,
as per the pshat of the mishna & gemora in Ketubos as referred to by Rav
Moshe, and not when in their home regardless of who is there).  In the
Satmar community where they shave their heads, no, I am pretty sure no women
are relying on this leniency.   Within the communities on the spectrum
between these two poles, I suspect it varies, getting more likely as you
move towards the more "modern" end and less likely as you move towards the
more charedi and certainly Chassidic end.  But Rav Moshe never having heard
of it in his and in previous generations is a notable data point.

Regards

Chana