Holy shit, man. I am very sorry to hear about the girls' mother, but as prevous posters have said... her decision (remember, it was her conscious choice to cope with her struggles in that manner) was utterly beyond your control.
All you can do is be the best father to those girls right now, and hope that they will understand the whole story once they are older and ready to hear the facts. Given what a cool dad you seem to be, I have a feeling they'll turn out okay.
And I'm glad it's working out with the former nanny... take joy where you can find it! If you want to start a family with her and those girls, no one will stop you. We will be here to cheer you on!
It's funny, isn't it, how life is really just made up of one seemingly random event after another, and how we react to those events. Thinking about your story, there's just so much that "happened" to you (it wasn't planned ahead of time)... and yet you have been able to roll with pretty much everything. That takes serious character and integrity.
Good work, Chris.