I sincerely hope you won't encourage your aunt to divorce her husband, even if he is selfish and difficult to live with. Jesus said, "Therefore what God has joined together, let man not separate" (Matthew 19:6).
Remember, when your aunt and her husband were married they took a vow before God to be faithful to each other for as long as they live, and God takes that vow very seriously. I'm afraid we often lose sight of this today, and have come to look on marriage as a casual arrangement that can be ended whenever we please. But when we do, we pay a price -- a price of insecurity and instability and hardship. I know that sometimes separation or divorce become necessary, but every effort should be made to prevent them.
What can you do? First, pray for your aunt -- and also for her husband. Pray not only that they'll manage to get along with each other, but also that God will transform their lives and their marriage. And this can happen, as they turn to Jesus Christ and invite Him to take control of their lives. God can take away our selfishness and insensitivity, and replace them with His love and tenderness.
Then ask God to help you be a friend to your aunt. People in her situation often feel isolated and lonely, and she needs to know that others care. The Bible says, "Carry each other's burdens, and in this way you will fulfill the law of Christ" (Galatians 6:2).