D.H.
Dear P.,
I've hear what you say, it is so tough being Mom and a Step Mom, with young children. And I read all the advices from several other Moms. 5 years old that is still so young to reason like an adult. There is a lot of anymosity, and I mean lots. The 5 year old as you put it is definitely lashing out, that is clearly a cry for help. What do the counselors say about this?
A suggestion is to have the father plan special short outings individually with each child, so they know that he still loves them as much as he does you and the new baby. Find any instances of good behavior and really build on that so to promote good behavior and self esteem. Make sure you get some space and time for yourself so that you can handle the tension bad behavior brings into the family.
Watch the movie, "The Secret", and find some positive motivational CD's that you can listen to during the day, (won't hurt the children either), that will help you during times of stress and to refocus.
And P., you are really doing good, you are trying so hard to find an answer. I admire you so much! God Bless You and Hang In There! D.