husband sleeping with his sister?

I am pregnant with our first 2 babies, and I just found out my husband is sleeping with his sister. I came home early today from work (I only nanny one day a week) not feeling well and caught them in bed together. He owns a business and works from home. I want to leave but he said he won't do it again and honestly I want to be able to be a SAHM to my twins, and my husband makes enough money to support all of us, and I've never had a real job. I just feel like it's wrong to make my twins possibly live in poverty, as I'm a housewife, and he makes over six figures... I'm so upset and I don't know what to do  I'm so disgusted. And yes providing a good life for my babies is more important than anything else. I do believe he should seek counseling. And no, both him and his sister were adopted at 8 and 10 so they're not biologically related. Also: I'm a housewife, but I nanny one day a week (my only work experience by the way) to clear that up. I love this man more than anything, I believe in forgiveness, and I don't want to live my life without him.