All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
It is incumbent upon the wife to adhere to the condition that she had agreed to upon the conclusion of her marriage contract since Islamic Law commands Muslims to adhere to contracts and their conditions. Allah, The Exalted, Said (What means): "O ye who believe! fulfill (all) obligations" {Al-Ma`idah/ 1}. In addition, Prophet Mohammad (PBUH) said: "Muslims will be held to their conditions, except the conditions that make the lawful unlawful, or the unlawful lawful." {At-Tirmizi}. The husband`s second marriage doesn`t annul the conditions that he has stipulated in his first marriage since they are two separate contracts, and each has its own conditions that should be adhered to. If your husband had stipulated that you should live with his mother, and the house is large enough that you can enjoy your privacy, then it is imperative that you adhere to this condition, and make no objection under the pretext of his second marriage.
However, if the house was small, and you couldn`t enjoy your privacy, then you have the right to live in a separate one. Rest assured that you have engaged in an act of kindness once you agreed to your husband`s condition; therefore, deny not yourself the reward of this good deed, and break not the thread of love between you and your husband, whom you have referred to as God-fearing and modest, since this is a grace that not many a wife has. As for his other marriage, there is nothing you can do, but to accept it. And Allah Knows Best.