9 year old is the most difficult...

I am a mom of 3 boys ages 9, 7 and almost 2. My 9 year old is the most difficult for me to deal with. He is extremely oppositional. He fights with his 7 year old brother much of the time. The 7 year old has come to exhibit a lot of his defiant behaviors as well. They both seem very angry. To make matter worse, their dad, my husband, was diagnosed a year ago with Alzheimers Disease, and I feel like a single parent these days. He has a very hard time dealing with their defiance and battles with them, so that sometimes I feel like there's 4 kids instead of 3! I love them, but feel like running away these days and never looking back. I'm terrified of how they'll be when they reach adolescence, if I have little control now. I've had my oldest in therapy off and on over the years, tried various medications. He's very smart and popular at school and behaves well at school. I feel like a failure, because their behavior seems reserved ! mainly for us. What can I do? This seems like an impossible situation!