Is it just common for the new-step mom to alienate the ex-wife?

You know how the saying goes - what goes around comes around. Well, when I married my now ex and became a step mom - I detested the ex wife/mother - I suppose a spousal siding thang. Now that the tables have turned and now my ex has remarried and the new wife/step mom is hell bent on hating me - I taste the medicine I dispensed. Now that I am a Mom, I now understand and have concerns; my intuitive son has picked up on it. How will this effect him in the long run? The Ex and new (4Th) wife are a combative team to destroy me and my sons relationship. They, in short, are out to replace and eliminate me. I've got to wonder - is it this just common day natural occurrence / given?