Bob,
I brought into my marrage a 6 year old boy...my hudband had no children and he loved my son with all his heart. But the bottom line...and this is where probably 100% of our problems stemmed is that he was MY son. I said all the right things to my H but I would end up resenting and then stepping into the middle almost everytime my H would correct my son. Even two parents of a natural child have conflicts about how to raise them but it much more touchy when one is a step. I really don't think there is anyway to change that. It is just one more thing over which we have no control. So perhaps take a step back and let her have the final say with regard to the kids...if you have not alreay done so. I am sure she will consider your feelings.
Secondly, you already know that 4pm is a zoo so maybe try to NOT work at that time and return when the kids have gone to bed. I have had resentments as well when all hell is breaking loose and my H is not available (walking??) to be there. Perhaps cooking and feeding the baby so she can handle the rest might be a thought. I bet you will find that by 7pm things have calmed down enough that you can get back to work.
I am concerned that your frustration and her obviously growing resentments are threatening both of your sobriety.
In my journey I have dicovered that the tone of those first few moments when you get back together at the end of the day pretty much determine how the rest of the evening will go.
Best of luck,
JT