Gosh, I have a policy against getting personal here, especially since you're geographically close to me...
I'm not being unfriendly, but it's a little creepy... I'm sure you get it.
But, let me offer this.
My D18 is my H's step-daughter. Her dad and I were divorced when she was littlelittlelittle. I've dealt w/him for years, as far away as I could possibly do it, considering our large, but small community.
I want you to know, based on dealing w/kids and divorce, that it's one thing to deal w/their questions and comments, but it's another to fuel their fire.
ACT AS IF it's totally okay with you to be separting w/their mom. ACT AS IF it's an okay thing to deal w/separation and divorce. I, personally and professionally, have several friends who are dong a SUPERB job at co-parenting. So, lead your STBXW into that finality. Your girls are so young. So precious. If you can get past the hurt and anger at her actions (and, trust me, I understand your angst) you will be so much better off for them.
YOU will find YOU with someone else. We are not such a small pond, here.
Just my thoughts. You seem like a really caring father. Now be the DAD they need!
HUGS
Me-46, D-21, S15, S13
After many years w/my head in the sand... I FILED Divorced 6/2011
The average woman would rather have beauty than brains, because the average man can see better than he can think.