Children are alot more resillient than we often think. Children in most cases should be told the truth. You tell your son just enough so that he does know what is going on, simplify the details and do not allow your anger to take over. Even if your son was never told of anything previously, I'm sure that you realise he can and does see things that are taking place in your home between your husband and yourself, and not having anything explained properly to him can cause him to be stressed and confused. He obviously can see that this situation has made you an unhappy person and it is natural for a child of his age to think that his half sister is to blame.
You say that you had " accepted this child before and were willing to be there for her until her mother started to harass you? BUT in your first post you say, " one of the conditions for us getting together was that he should cut all contact with this woman and that I do not want this child in my life".
In honesty, I believe that what you said about not wanting the child in your life was what you really thought, and it is understandable that you are angry, but you really should have given this situation alot more thought before you made any decisions, and as for the other woman, her knowing how much yu are against her daughter and your husband seeing each other is going to make her determined that you do not destroy their relationship.
Someone has to take the first step, and if you really wish to make your marriage work, why should it not be you? You all cannot just live a life with mistrust and hostility, and for the sake of all the kids you all have to come to some reasonable solution. These kids are already being used as 'dispute settlers', and that will not have good results.
Make the appointment to see the counselor and tell yur husband the time and date, if he is serious he will go. As I said previously, you forgave him and he chose to be with you, and for the happiness of all the children involved do not allow things to continue in the way they are now, get the help that you all need to overcome and settle this matter.