J.,
I think it is common for men to take for granted that we just take care of the kids and can do it all on our own. My husband is this way at times. Seems like he thinks it is just "my job". I have talked to him and told him that people who work a regular job do get days off and lunch breaks, and that I need that once in a while too. Being a mom is a 24/7 job and it really can wear you out! Especially when you are the only one home with them all week long, you do deserve a break and a little help.
I suggest taking time to talk to him when he is not already in the middle of one of his own "more important" projects and at a time when you are not totally stressed! Maybe in the evening you could ask him if he would set aside some time the following day to watch the kids while you do something on your own (even if it's just going to the grocery store alone, it would help alot).
Also, do you have another family member or a friend who could watch the kids for you once in a while during the week, so you can have a little time to yourself? This might help ease your stress, so that you are not so frustrated by the time your husband is home on the weekend. I used to feel guilty getting a sitter when I am able to be with my kids or take them with me, but sometimes just an hour or two alone makes me feel like a new person.
I hope this helps a little.
As for your step daughter, I am wondering why you have custody of her? If her mother wants her, was there a reason your husband was granted custody over her mother? Do both of his children have the same mother? (if so I wonder why they are not both in the same home) Without more information about this situation, I would not want to offer an opinion about her going back to her mom. All I can say is I hope you would do what is best for the child and not let your frustrations with your husband cause you to make a harsh decision that would affect this little girl forever.