My wife and i are both practicing muslims. My mother-in-law is a zoroastrian (parsi) and when she comes to stay at our house sometimes she performs her own prayers. Is this allowed in our muslim household?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Apparently – and Allaah Knows best – it is permissible to allow a non-Muslim to perform his religious rituals while he is visiting a Muslim, particularly if the non-Muslim is hoped to be attracted to Islam by this permission. In his book “Ahkaam Ahl Ath-Thimmah (i.e. Rulings of non-Muslims living under Islamic rule)”, Ibn Al-Qayyim said: "It was authentically narrated that the Prophet, sallallaahu ‘alayhi wa sallam, welcomed the delegation of Christians from Najraan in his Masjid, and when the time of their prayer was due, they performed it there. This took place in the year of delegations." Since this happened in the Masjid which is a place exclusive to Muslims' worship, they would with greater reason be allowed to do so in the Muslim's house.

However, you should demonstrate for this woman and other non-Muslims the beauties of Islam in acts of worship, dealings, etiquettes and manners which would encourage them to embrace Islam. For more benefit, refer to Fatwa 85180. 

Allaah Knows best.