Am I sinful for not speaking to my brother and his wife, knowing that the reason for the estrangement is my brother's wife creating problems, and my brother does not speak to me out of consideration for her, despite his admission that there is no animosity?

The Prophet forbade discord and estrangement among Muslims, especially among brothers. Maintaining ties with them is obligatory, and severing ties with them is forbidden. If you do not sever ties with your brother, but he severs ties with you, you are not sinful. However, you should strive to reconnect with him and remove the causes of estrangement, and be keen to do good to him even if he cuts you off. For the one who truly maintains ties is he who reconnects with his kin when they sever ties. And whoever pardons the one who wrongs him, that will increase his honor and earn him Allah's forgiveness.