My step mother is lovely. She has been my stepmother since I was 14.
My mother maintained very friendly relationship with my granny and with my aunts. She had been DIL for 20 years when my parents separated and was and is the mother of the grandchildren (including me) and nieces/nephews.
As a consequence she was often invited to family "dos" at my grandmother's house (I.e. Aunt's 60th, grandmother's 80th). And has also come to my father's 60th and 70th parties at his and step mother's house. My mother and step mother actually click quite well. They have lots to talk about- books and films for example, without ever talking about my dad/relationships and so on.
She came to my grandmother's funeral - and it was lovely that she was there. There were so many people there that she had known when she was married to my father - the friends of my younger aunts and so on.
My step mother is absolutely not second best. I can see that my mum is sometimes greeted as an old and rarely seen friend, and I can appreciate that that could make my stepmother feel sidelined. But it doesn't, because she is confident and clearly married to my father, and my dad and mum only really talk about us and the grandchildren. They don't click as well as my mum and stepmother do!
I know the separation was horrible for my mother (dad had an affair, not with step mother) partly because of the loss of my father's family (she actually was terribly fond of my grandmother - more so really than her own). So it is lovely that it has panned out that she still participates in some family events, is in regular contact with my aunts, gets on with my stepmother, and stepmother isn't threatened by this at all. I don't think it would have panned out like this if dad had stayed with the OW, however. I don't think that would ever have been comfortable for either of them.