DH and I have a 5-year-old son.
I also have 18-y-o twin sons, from a previous relationship. Their dad lives nearby and is involved. My 5-y-o has spent time with him and clearly understands that "Ex" is the twins' father, not DH.
DH also has a 14-y-o son, from a previous marriage, who lives with us. I don't think our 5-y-o has ever met the 14-y-o's mother, but he's acutely aware that his brother flies away to visit her, 3 times/year. He forgets sometimes that I'm not the 14-y-o's mommy...but if he thinks about it, he remembers.
Little did I realize how much MORE complicated the 5-y-o thought things were! Understandably, it's hard for him to wrap his mind around the idea that his father and I have not always been married; that we could possibly have had lives, before he existed. Tonight:
5-y-o: "So, are (the twins and the middle son) adopted?"
Before I can launch into an explanation, 5-y-o: "No, I remember. "Ex" (the twins' dad) used to have a different wife (than the one he has now) and they had (middle son). Then she moved to California and he married (twins' stepmom) and they had the twins. Right?"
So, not only does he think the 14-y-o came from Ex's first marriage, and the 18-y-o's from his second, but he imagined that all 3 of these other parents gave over their children, for DH and me to raise?
I explained how it actually is.
5-y-o: "Oh. Yeah, I knew that, about Daddy being married to (middle son)'s mom. I just didn't know the "Ex" part. That you were married to "Ex". I didn't know about that."
It was pretty obvious that for ME to have been with someone else borders on scandalous...but not so much, Daddy's prior marriage!