First I want to say your doing a wonderful job as a mother, sometimes it's often hard when a child lose their parent because they just don't want to be a mother or a father. I also have to say your holding on strong because you did not walk out of this relationship and you truly want your two daughters to grow up together I understand your point of it all, me as a stepparent understand how you feel.
I think the wonderful steps your taking to talk about it is a great step for you and your family, I hear your hurt when it comes to his family and that's truly wrong on their part, they should have open arms because your taking on the task her mother should have but she didn't she wanted to move to New York and not think about her daughter. The most important thing you and your spouse should work on is not fighting in front of the kid's, that's truly one thing they will remember. Both daughters have gain something from this relationship, one gain a mother because her bio mother went to New York and the other daughter gain a father because he went back to Canada.
Another thing that can help you as a couple, set a day and time to discuss issue by going out to eat without the kid's, this will help you and your spouse control those tempers because your out in public and also in a nice place eating dinner. Another tool to use when daddy and mommy temper go out of control is to make sure you both tell your kid's your sorry they have to see you guys fight like this, believe are not kid's remember this and it will stick in their mind and also this will help you and your spouse from fighting in front of the kid's.