Allah forbid, what if one day a kafir person asks me to accept someone else as my God, and that if I don’t, they will torture my brother. Wouldn’t I be the cause of the torture? And is it still haram to accept that person as my God, astagfirullah, if my only reason was to save the person from getting tortured?

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 (sallallaahu 'alayhi wa sallam) is His slave and Messenger.

It is not permissible to outwardly show disbelief in order to save one’s brother or anyone else from torture. The concession applies only in the case of coercion and fear for one’s own life, as Allah the Almighty Says (what means): {Whoever disbelieves in Allah after his belief — except for the one who is compelled while his heart is secure in faith…} (Quran 16:106)

Al-Jassās () said in his explanation of the following verse: {O you who believe, do not take My enemies and your enemies as allies…} (Quran 60:1)

"This verse indicates that fear for one’s wealth or children does not permit practicing taqiyyah (saying something contrary to what one believes, for fear of harm that may befall him if he does not resort to (taqiyyah) through outwardly showing disbelief. Similarly, it [also indicates that such fear] is not equivalent to fear for one’s own life. Allah forbade the believers from doing what Hātib () [Companion of the Messenger of Allah (sallallaahu 'alayhi wa sallam)] did despite his fear for his family and wealth."

He also said: "Our scholars have likewise stated that if someone is told: 'I will kill your child unless you disbelieve,' he is not allowed to outwardly show disbelief…"

Al-Jassās () continues by saying:"What also tells us that fear for wealth and family does not permit taqiyyah, is the fact that Allah made migration obligatory upon the believers and did not excuse them from it on account of their wealth or families.Hence,He Said (what means): {‘Say: If your fathers, your sons, your brothers, your spouses, and your relatives…} (Quran 9:24).

Similarly, He Said (what means): {They will say: We were oppressed in the land. They will be told: ‘Was not the earth of Allah spacious enough for you to migrate therein?’} (Quran 4:97)

Lastly, it is stated in Ḥāshiyat Al-Dusuqī () on Al-Sharḥ Al-Kabīr: "If a person is threatened with something other than being killed—such as being beaten, having his child killed, or his wealth looted — and he then insults Allah or the Prophet (sallallaahu 'alayhi wa sallam), he becomes an apostate.”

Allah Knows best.