You must be feeling so stressed with this big change happening right when you're caring for such a young baby!
It's good that you are looking at all of the issues and going into this with a good look at what is to come, instead of going into it blind.
It is also fair that the baby's needs need to come first. You can explain this to all involved by pointing out that he depends on you and his father for everything right now. But when talking about this, say, "That doesn't mean that x, y, and z's needs don't get met. Just like me, it merely means we have to be a little flexible. In this (whatever comes up) situation, I'm going to be flexible and do yada yada. Girl A, can you be wonderful and do yada hoo, too? Girl B, can you be sweet and do blah blah blah? This way, we're a team."
And you know what? Here's where you win the lottery. I bet those girls are going to love your new baby, and will help out a ton. As long as you water that garden! Allow nature to take its course and don't interrupt with your fears and such. Because here's the math: Whatever extra work their presence creates will be canceled out by all the help and playtime they will give him.
I say, talk to your husband. Divide up tasks. Where he wants to put work on you, hold his hand, wade him into the water. He'll take over there as his comfort level grows.
Also, bear in mind that although you are not their biological mom, you are their father's wife. You had some time with your husband without the girls. I think that it makes sense that they should spend some time living with him, too. If you see this as an opportunity at least as much as it is a challenge, your feelings of stress won't turn into resentment, and that won't then turn into an automatic unhappy family. Those girls aren't all that familiar with you. They might see you as a cool older friend, if you present yourself that way. You might really enjoy them, if you allow yourself to.
About the ex being controlling: see this as a sign that she really cares. It might mean that she is an anxious person. Try viewing her through a lens of compassion rather than fear - your fear and stress are making her seem controlling to you rather than as having issues of her own. You can manage how it feels in your life, and how she responds to you, by calming her fears - *especially in the beginning* - by emphasizing what you agree on, and minimizing what you disagree on. When she tells you to do things that you think are over the line, diplomatically say, "Thanks for the suggestion. I'll definitely have to keep that in mind!" Doesn't mean you have to follow through. When she says things that you think make sense, really calm and stroke her ego (we all have one) by saying, "wow, yeah, I completely agree 100%. It's so important that the girls blah blah blah."
WHen there are disagreements, start out by saying, "I really like when...." fill in the blank. Then say, "We both want what's best here. I can see that you want/need blah blah blah. Let's make this work by doing it like this: blah blah blah. What do you think?"
If you're not a SAHM, consider going SAHM. This way, the girls are in school all day and you're home and able to get your time in with your little boy. Then, after they come home/you pick them up, they play with him, you help with homework, and they feel their needs are satisfied, too. And trust me, they will do their own thing, too, and let you do yours. You will also have more time and energy for your husband.
Don't forget sex. Sex is a wonderful way to re-create bonds with your spouse. And to ease any resentment that he is starting to feel. It makes those difficult conversations so much easier because everyone is more relaxed :) and loving.
Good luck. I think you'll do great.
L.