Ruling on joining non-Muslims in their prayers
9-5-2023 | IslamWeb
Question:
My dad and i are muslim and my mum is christian. I still live with my parents and my mum always asks me to pray with her in the mornings. Is it permissible to do so?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
It is not permissible for you, dear questioner, to join your mother in her prayers for several reasons:
First: Allah the Almighty is not worshiped except by what He has legislated, and the prayer of Christians is not of what Allah The Almighty has ordained, and the law of Islam overrides all previous laws, so it is not permissible for a Muslim to worship his Lord with any of the legislations of the previous prophets.
Second: Their prayer is not devoid of associating partners with Allah the Almighty, as they worship Jesus and his mother, so participating in their prayer is disbelief in Allah the Almighty, and Allah forbade us in His Book from sitting with the disbelievers when they disbelieve, so joining them in their acts is graver for a greater reason. Allah the Almighty says (what means): {And it has already come down to you in the Book [i.e., the Quran] that when you hear the verses of Allah [recited], they are denied [by them] and ridiculed; so do not sit with them until they enter into another conversation. Indeed, you would then be like them. Indeed, Allah will gather the hypocrites and disbelievers in Hell all together.} [Quran 04:140].
Shaykh Al-Islam Ibn Taymiyyah 
said in Majmoo' Al-Fatawa: “If entering for leisure or something else is forbidden because wrath descends upon them, so it is forbidden for a greater reason joining them in their religious rituals which incur Allah's wrath against.
Third: Joining them in prayer, even if only outwardly, without repeating their sayings, is the ugliest type of imitation because it is imitating them in what is part of their religion, and the Messenger of Allah said: “...Do not imitate the Jews and the Christians.” [Ahmad, At-Tirmidhi, and Ibn Hibban]. In another hadeeth, it is stated: “He who resembles any people is one of them.” [Abu Dawood and Ibn Hibban]. Shaykh al-Islam said in Majmoo' al-Fatawa: “If a Muslim were to imitate the Jews or Christians in any of the matters related to them, it would be forbidden, according to the consensus of the scholars. And it is forbidden for a greater reason to join them in their traditions and religious rituals.”
Finally: we advise you to strive hard in inviting your mother to Islam so that Allah the Almighty may open her heart and save her from Hell through you.
Allah knows best.