My fiance's little brothers (from his dad's 2nd marriage) are young enough to be his kids, and I kind of wish they WERE his sons, not his brother, sometimes. I feel like I brought 2 kids from previous relationships into this, and it would, on some level, be cool if he did too. Kids are great, no matter where/who they came from, you know?
Your situation is somewhat similar to some of our friends. They had been married for 4 years when he found out he was the father of a 14 year old girl. Obviously, this had happened well before our friends met and married. His daughter is now 17 and the couple is pregnant with their 1st child together. Over the years, they've actually grown to know/love each other as family (think of how hard that was on the 14 year old to finally know her dad!)... theirs is a 'happily ever after' story for a similar situation.
If I were you, and your husband, I would want to know FOR SURE if these are his kids, if for no other reason than to be able to tell the children you have together that they have half-siblings. If you don't know, you won't be able to answer any questions they may have.
One more little side note, my 5 year old has a half-brother that she's met ONCE when she was 2, and he lives states away. We'll probably never see him again unless he makes contact (or she does) when they're older. I'd love for them to know each other, but if that's not their choice, that's fine too. I guess my point is, I'm honest and open with my daughter if she asks about it, but it doesn't effect her, and that's something you worry about.
Hope it gets better!! Follow your heart and gut on this one, and I applaud your hubs for being honest with you. You're taking it like a champ :)