My main concern is that mortgage payments contain (Riba) interest, I wanted to know if it is ok for me to pay for my brother's mortgage for his house from the money provided by him, also keeping in mind that I don't live in that house. Please let me know if I am incurring any sin?

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 unlawful as it represents a help in sin. It is a rule in Sharia that anything that contains a help in sin or transgression is forbidden as Allah Says (interpretation of meaning): {… Help you one another in AlBirr and AtTaqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allâh. Verily, Allâh is Severe in punishment.} [5: 2].
Allah knows best.