Children & Step-Children, What Do They Know?

I called my children this morning. While catching up on all the xbox 360 news with my son who is 10, out of the blue  he said: "Mum, you did not told me that Papa (Dad) was married to Liz!" "Well I thought you knew, I mean your brother and sister live with her... Isn't it obvious that she would be their mother?" "Well... yeah... but I thought Alex came out of you! And Stuzz too.  I did not know that Liz was their mum." I did not know what to say really. I thought it was obvious. I never thought about having to explain that to my children, nor did they even ask or questioned the situation.  We basically always had a good relationship with his ex-wife. Of course there was the occasional arguments between my ex and his ex but other than that, she would stay at our place with her husband and vice versa, we spent most Christmas together too and other family related celebrations. I would pick up my step children most weekends, most school holidays and even though it was an hour drive (100km) to get to them I would even go down anytime needed even just for the day.  She also has a son with her current husband. Her son is a few months older than my daughter. I remember going to the hospital for support, pregnant with my daughter, because her newborn had to have an open-heart surgery. My kids consider him a brother too and will refer to him as "their brother". So I was a bit surprised that my son put 2 and 2 together just recently.  My ex-husband's ex and I have been very good friends for years and she is probably the closest family my kids ever had.