Have been following this post for a while, I'm new on here, a BM with DD9 and I have to admit I was facing/am still facing similar problems to the OP.
Just a couple of minutes ago, I read a thread on another SM forum, just from the other side's point of view ... a boy asked his BF and SM whether he could bring his half-sibling (BM's son from new relationship) to stay for next weekend contact. SM explained to SS that this was not possible, "because he is not related to us". Funnily enough, there was not outcry condemning this statement.
Many of my SS friends get very upset when they are an issue of discussion between BM and BF (well, you discuss BM with DH, right?) or at the thought that stepchild takes home photos of SM and her birthchildren to Mum (again, you have photos of BM's child and possible some old ones of her). Same thing!
During the first couple of months after our separation, I gave a lot of thought to this issue ... joined SM forums, read SM books to see the other side. Now, three years on, I have learned that "family or not family" are as difficult to define as "the best interest of the child".
I ultimately decided not to go with the "blended family" approach a la Brady Bunch that my ex and his new wife are following rigorously ... because I noticed fairly soon that whilst it was expected that SM joins "my" family, I wasn't very welcome to join SM's (by the way, I have never met SM or spoken to her).
So the first time SM attended DD's school, I announced to ex that I will attend her daughters' school the following week ... after all, they are my DD's stepsiblings, no? We have since a consent order (rather grudgingly though on behalf of my ex) that stepparents (from neither side) do not get involved in education, medical issues or issues of appearance.
Just food for thought ... because no matter how unreasonable you consider the "other woman" (old or new), doors would need to open both ways for this to work.