N., I think it's very admirable that you have chosen to co-parent your husband's son with this woman. It shows your compassion for your husband, the little men, the situation overall AND their mother!
I do think that you should attempt to speak with the birth mother, however, you should make sure of some things first. Make sure that you've discussed with your husband what you're going to bring to the table & that you agree on the approach. Also, be sure of what it is you'd like to communicate to her...what, if anything, you'll be asking of her to contribute...and most of all, how you'll handle yourself if things become heated.
It is not crucial for you and the real mother to get along, however, I'm sure you know, that it will make the situation much easier to deal with for everyone.
The most important parts of this situation is the little men and that they are not (further) traumatized by the situation. Ultimately, if you care for them as you seem to, you just need to make sure you support your husband and that you two do all you can to make sure that they know they are loved.
Blessings to your family.