Liesagain,
Well crap......I know you thought you were doing the right thing, but now you know, so time to move on with your plan of action.
As far as the step-child goes, if the law allows and you can afford to keep the child (who may be not that young from what i gather?), then let them stay with you and your kids.
You'll need to file a restraining order ASAP against AH. Do it today before something bad happens.