I agree with all that has been saisd here. But that doesn't matter. What matters is how much your heart can hold. This is a huge commitment you are considering. It is a bigger commitment than the one you are planning to make to your fiancee. You are planning to become a mother to two emotionally needy children. The only piece of advice I'll throw in here is this:
If you do it ... You need to do it 1,000%
Don't take these children thinking in your heart you'll "try it". No.
If you take this little girl, ...if you take this little boy, then it will not be ok to say..." I've changed my mind, this isn't working out."
Take them into your heart as if you have just given birth to them.
Take them only if you are willing to become their mother in your heart.
Because in a situation like this, it won't work for you or for them otherwise.
Commit to it , or don't. Either choice is OK. But make that choice now, for their sakes.
Only you and your fiancee can decide what your hearts and arms can hold. But speaking as a mother... It's not hard at all, to love a child. Especially one who needs you.
Val