Step Families Support Group
Traditionally, a stepfamily is the family one acquires when a parent enters a new marriage, whether the parent was widowed or divorced. This community is focused on many of the relationships and issues that may arise within stepfamilies where you may find answers from some members and you may help other members with their questions.
The father's day fiasco is a perfect example of this. While he may not have been privy to their plans to have an all-out bash, to which you and your children were invited, he must have figured it out while it was going on. "I thought it was just going to be you and me...". The fact that he failed to stop this obvious affront is very telling about how much he values your relationship; whether he just doesn't or that he can be manipulated into hurting you so badly, is neither here nor there.
The bottom line is, he seems to be able to live with himself knowing that you and your children are outcasts in your own home; I did not say that he was comfortable with it, I just said that he is able to stand idly by as it happens.
My advice to you is to have an open and honest discussion with him about the nature of your relationship, without his family or your family around; while his family is a large component of the problem, the real problem is really between the two of you. If at all possible, perhaps an impartial third party (counselor/minister/etc.) could act as a mediator. When having this discussion, it is best to use "I" statements, rather than "you" statements. "I felt hurt, unvalued, and excluded when I was not invited to the father's day celebration." might be better than "Your family had a party for you, and you let it happen, knowing that I wasn't allowed to come. That really hurt."
If he is willing to have this conversation with you, and start making some changes, then the two of you might just come out alright. If he is not willing to have this conversation or make changes, then, I'm afraid, I cannot see a good reason to continue to subject yourself or your children to this sort of treatment.
I know that a father's first duty is to his children and a spouse second, but if he is allowing his children/family/ex to behave in such a malicious way at your and your children's expense, he is not helping anybody; he is teaching them that malicious behavior is acceptable.
Please keep us apprised of the situation, and know that I am praying for you.