Hi John. One of the reasons that I don't post detailed tree information on the internet sites; they don't seem to have the finesse of "odd" relationships.
I use FamilyTreeMaker on my PC. When I have had a situation like yours, when a person re-marries, the programme asks if I want to "attach" the children to the new partner, which I answer "no", as they are not his/her children (they are step-children)

Solves the problem!