She wore niqaab during Hajj; is there any sin on her?

1 – Covering the face is something good for women. May Allaah
bestow abundant mercy on the women who cover their faces with niqaab.
Perhaps your question is based on keenness for your religion – may Allaah
increase your keenness – but it is better to heed the commands of the
religion, which tells women to remove the niqaab from their faces during
prayer and Hajj, but especially during prayer, when it is not permissible to
cover the face at all except if there are non-mahram men present.

During Hajj it is permissible for women to lower the veil over their faces
provided that it does not cling to their faces; this is what is called the
burqa’. It is not permissible for the woman in ihraam to wear it, but it is
permissible for her to lower over her face a cover which comes down from her
head over her face. If the burqa’ is not available, and there are men
present who should not see the woman’s face, then it is permissible for her
to wear the niqaab. 

It was narrated that ‘Abd-Allaah ibn ‘Umar (may Allaah be
pleased with him) said: “A man stood up and said, ‘O Messenger of Allaah,
what kind of clothes do you command us to wear during ihraam?’ The Prophet
(peace and blessings of Allaah be upon him) said: ‘Do not wear a shirt
or pants or a turban or a burnous. If one of you does not have any sandals
then let him wear the khuffayn (leather slippers) and cut them so that they
come below the ankle. Do not wear anything that has been dyed with saffron
or turmeric. Women in ihraam should not wear niqaab or gloves.”

(Narrated by al-Bukhaari, 1468; Muslim, 1177). 

2 – With regard to lowering a cover over the face other than
the niqaab, or for fear of being seen by non-mahram men, it has been
reported in saheeh reports that some of the Sahaabiyyaat (women of the
Sahaabah) did that: 

It was narrated that ‘Aa’ishah said: “We used to go out with
the Messenger of Allaah (peace and blessings of Allaah be upon him)
when we were in ihraam. If we met the riders we would lower our garments
over our faces.

(Narrated by Abu Dawood, 1833; Ibn Maajah, 2935) 

Shaykh Ibn ‘Uthaymeen said: 

It is not narrated that the Prophet (peace and blessings
of Allaah be upon him) forbade the woman in ihraam to cover her face; rather
he forbade the niqaab only because it is worn on the face. He differentiated
between the niqaab and covering the face. Based on this, if a woman in
ihraam covers her face, we say there is nothing wrong with that, but it is
better for her to uncover it so long as there are no non-mahram men around
her, in which case she should cover her face from them. Al-Sharh
al-Mumti’, 7/153 

3 – With regard to the past and the Hajj that you did in the
manner you describe, there is no sin on you because you were unaware. If a
person does something that is not allowed during ihraam because he was
unaware or he forgot, there is no sin on him, and he does not have to offer
any fidyah (ransom). 

It was narrated from Ya’la ibn Umayyah (may Allaah be pleased
with him) that a man came to the Prophet (peace and blessings of Allaah
be upon him) when he was in al-Ji’raanah and entered ihraam for ‘Umrah. The
man had dyed his beard and hair with saffron, and he was wearing a jubbah
(cloak). He said, “Take off your jubbah and wash off your saffron, and
whatever you do in your Hajj, do in your ‘Umrah.”

(Narrated by al-Bukhaari, 1697; Muslim, 1180). 

Shaykh Ibn ‘Uthaymeen said: 

Similar to forgetting are cases where one is unaware or is
forced to do something, i.e., if a person forgets and puts on ordinary
clothes when he is in ihraam, there is no sin on him, but when he remembers,
he has to take them off and put on the izaar and rida’ (ihraam garments).
The same applies to perfume; if he puts on perfume, forgetting that he is in
ihraam, there is no sin on him, but when he remembers, he must hasten to
wash it off. 

Al-Sharh al-Mumti’, 7/222

 And Allaah knows best.