littlebit--
Be strong.
He said he knew he was going to lose some blood family over this but he felt that strongly about her. This isn't fog talk is it?
In a way, I think it is. I don't think he's looking far enough down the road to see the lack of future his relationship with the OW has.
You said "everyone knows" -- does the OW's family (mom/dad/brothers/sisters) know? Is the OW prepared to "lose some blood family" over this?
How are your friends (of both you and your husband) reacting? Are they trying to stay neutral? If so, they're not really great friends, in my opinion.
Enlist all support you can to apply some pressure to him to stop acting like a selfish child and drop the OW.
You need to decide if you want this marriage. Seriously. You need to make a choice. One of the biggest mistakes I made after my wife's first affair was NOT giving divorce a serious and deserved consideration. I knee-jerked the "I'll fight for my marriage" response and allowed my wife to sail along until her second affair. We're still struggling, but the second/third time around, I gave divorce a more serious consideration.
Since you're raising your sister, it is more complicated. If it was just you and your husband and no children, I think divorce and recovery are 50/50, especially since your other posts mentioned he was unfaithful to you before your marriage.
But, you and your sister can survive without him. The choice is yours.
If you decide to fight for recovery and your marriage, you need to read up on Plan A -- plan now for a strong and dark Plan B. There are many on here who can help you prepare for a Plan B.
Be open and honest with your sister and family. Keep open dialog with his family if they support you and your marriage. Encourage them not to coddle or accept his behavior.
Be strong.
Blessings.