3 years
That's not an awful long time, but I know alot can happen during that time, too. Marriage can be uphill battle sometimes, but if you're with the right person those struggles can sometimes actually bring you closer and make you a stronger couple.
Having a child with her also makes it very challenging to just walk away. Divorce IMO should be your very last option. It's just so hard on the kids and it will be hard on both of you, too. It can literally devastate your life, health, finances, etc.
Marriage counseling is a good option if both of you are open to the idea. You can also learn how to deal with her better and learn techniques to respond to her when she gets all up in your grill. It takes practice, but it can pay off big time.
I will be praying for you brother and hope you can somehow figure out how to make it work. As I mentioned earlier, the struggles you go through together can make your marriage stronger in the end. Have faith and don't give up!
Take care and God bless you man.