No, you're not alone!I had twins with my high school sweetheart shortly out of college (we weren't married). Now my husband (different guy) and I have a baby together. My ex and his new wife are about to have a baby, so the twins will have half-siblings in both homes.
Furthermore, my husband has custody of his son from a previous marriage, so the twins have a step-sibling at home, too. My husband and I get along so well with my ex and his wife that she acted as godmother for my step-son when he was baptized (obviously not as a baby). My step-son refers to my ex and his wife as his godmother and "step-dad" (which is puzzling to people who know that his actual mother is not remarried...;-) )
Additionally, my husband was married before, in college. It was kind of a sweet, idealistic thing that didn't last, where to some extent he felt he was rescuing a childhood friend, whose young son's dad wasn't in the picture. So, my husband ALSO has that step-son (for whom he's been the only father) and a biological child, with that first wife. Both of them were college-age by the time we got married.
Also, my ex's parents were divorced, so the twins have his dad and step-mom as one set of grandparents, and his mom, as well.
I have long ago accepted how "normal" a lot of this stuff is today and I spend my time trying to make the relationships civil and functional, rather than feeling guilty about te "broken homes". Yet, I still hope to raise my kids to have one marriage that lasts.