Are we allowed to pray sitting when sometimes going for picnic in a park so people will not see the sisters praying outside especially when a sister is wearing Niqab and she doesn't want to attracted people's eyes on her and that is why she prays with the Niqab on as well.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is forbidden for both men and women to perform obligatory prayers while sitting unless he/she is unable to perform them while standing. This is according to the Hadith narrated by Imran Ibn al-Hussain (may Allah be pleased with him) in which he said that The Prophet (peace be upon him) used to pray while standing, if unable, then sitting, if yet unable, then lying on his side. [Narrated by al-Bukhari ] As for supererogatory prayers, then both men and women may perform them either while standing or while sitting. Performing them while sitting, however, involves half the reward granted to performing them while standing. This is according to the Hadith: He who performs prayers while sitting gets half the reward given to performing them while standing. [Narrated by al-Bukhari and others] For a woman being in a public park is not an excuse for performing obligatory prayers while sitting, rather she has to pray in a hidden place as far as possible. She may pray in the presence of alien men even if she is noticeable, provided that she gathers herself (bring her body parts close to each other) while kneeling and prostrating. The women of the Prophet's (Sallallahu Alaihi wa Sallam) companions used to attend prayers in the Prophet's Mosque and in the Eid Musalla (prayer ground) in spite of the presence of alien men.

Allah knows best.