Is it obligatory for me to pray in the mosque in congregation and leave my brother to pray alone, or to pray with him in congregation in our place if he cannot leave the place?

The obligation of congregational prayer in the mosque is a matter of scholarly disagreement. Some scholars have favored its obligation for the able-bodied man who hears the call, unless there is an excuse. Therefore, your brother's inability to pray in the mosque due to an excuse is not an excuse for you, and the ruling remains applicable to you. If your brother was accustomed to praying in the mosque and was prevented from it due to an excuse, he will receive the reward for what he used to do, for the Prophet, peace and blessings be upon him, said: "Indeed, when a servant falls ill or travels, the like of what he used to do when he was a resident and healthy is recorded for him as reward."