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You need to be honest about how it hurt you, if you sit back and not say anything this type of behavior is going to continue. It's almost as if they are dismissing you as being part of the family now. Maybe talk to your husband and tell him you are tired of being treated this way and that you do not derserve such treatment. You should have been asked to stay and then they include the husband's ex wife. The Bride and Groom probably didn't want to leave anyone out and didn't want to feel like they were taking sides. If you don't like the treatment you have been receiving then you have to voice your feelings either to your husband so that he can say something to them or to them yourself.
You can't really make them stop or make them resepct you, you can only tell them that these things that they do hurt you and that you are part of the family now and want to be included in things that your husband is invited to. I think the reason they asked you step down is because they wanted their parents in the picture by themselves and that if you were included in one photo then that should have been good enough for you but what they should have done was kept you in the picture and included their real mother. I really think your husband should handle this situation seeing that it is his family that is doing this to you, you don't really want to seem like the bad guy or seem like you are trying to start trouble witnin the family but you shouldn't let them keep getting away with that. I think if they realize that their son is noticing how they are treating you they will try harder to include you in things.