Relatives before Whom a Woman does not need to observe Hijab
Allah says:
ﻻ جناح عليهن في آبايهن وﻻ أبنايهن وﻻ إخوانهن وﻻ أبناء إخوانهن وﻻ أبناء أخواتهن وﻻ نسايهن وﻻ ما ملكت أيمانهن
It is no sin on them before their fathers, or their sons, or their brothers, or their brother's sons, or the sons of their sisters, or their own (believing) women, or their (female) slaves. And (O ladies) have Taqwa of Allah. Verily, Allah is Ever All-Witness over everything.
When Allah commands women to observe Hijab in front of men to whom they are not related, He explains who are the relatives before whom they do not need to observe Hijab. This is like the exceptions stated in Surah An-Nur, where Allah says:
وﻻ يبدين زينتهن إﻻ لبعولتهن أو ءابايهن أو ءاباء بعولتهن أو أبنايهن أو أبناء بعولتهن أو إخوانهن أو بنى إخوانهن أو بنى أخوتهن أو نسايهن أو ما ملكت أيمنهن أو التبعين غير أولى اﻻربة من الرجال أو الطفل الذين لم يظهروا على عورت النساء
And not to reveal their adornment except to their husbands, or their fathers, or their husbands' fathers, or their sons, or their husbands' sons, or their brothers, or their brothers' sons, or their sisters' sons, or their women, or their right hand possessions, or the Tabi`in among men who do not have desire, or small children who are not aware of the nakedness of women. (24:31)
The Ayah contains more detail than this, which we have already discussed in the Tafsir of the Ayah and do not need to repeat here.
Ibn Jarir recorded that Ash-Sha`bi and Ikrimah said concerning the Ayah,
ﻻ جناح عليهن في آبايهن
(It is no sin on them before their fathers...),
I said, "What about the paternal uncle and the maternal uncle -- why are they not mentioned!''
He said: "Because they may describe her to their sons, so it is disliked for a woman to remove her covering in front of her paternal uncle or maternal uncle.''
وﻻ نسايهن
(or their own women),
means that they do not have to observe Hijab in front of other believing women
وﻻ ما ملكت أيمانهن
(or their (female) slaves.
Sa`id bin Al-Musayyib said:
"This means female slaves only.''
This was recorded by Ibn Abi Hatim.
واتقين الله إن الله كان على كل شيء شهيدا
And (O ladies) have Taqwa of Allah. Verily, Allah is Ever All-Witness over everything.
means, and fear Him in private and in public, for He witnesses all things and nothing is hidden from Him, so think of the One Who is always watching