I Think My Stepson Is Actually a Neighbor’s Biological Child. What Should I Do?

nytimes.com:

My husband and I have been married for four years, and together for over 10. Two months before our wedding, and shortly after he and his two children moved in with me to the house we bought together, I received a message from someone informing me that my husband and his ex-wife had been having an “affair” during our entire relationship.

We have received counseling and are working through it. That’s not my dilemma; I give this background to help explain how vehemently I hate this woman.

And, during their marriage, he caught her cheating on him multiple times. She was/is an alcoholic and prescription-drug abuser as well. (I know, what’s the draw?) I have always believed that their youngest child was a product of one of these affairs, and as the child gets older his resemblance to their old neighbor is becoming very striking.

This son is now 15 and has both emotional and physical problems, many due to his mother’s fentanyl overdose when pregnant. I had a conversation with my (now) husband early on in our relationship about this topic and he said that since he had raised this child as his son, at this point the child was his son and genetics didn’t matter.