Thanks again Renee for your help.
I can see things will get complicated when each partner to a marriage/partnership has previous or subsequent marriages/partnership and certainly if they have more than 1 previous or subequent marriage/partnership.
I will look at what you suggest.
But I wonder if my problem is that the way I am thinking of things in my example is that Parents 1, 2 and 3 can never be descendants. They are ancestors and their children are their descendants.
Obviously Parents 1, 2 and 3 are descandants of their (ancestor) parents who, in turn, are descendants of their (ancestor) parents.
But in considering just two generations is it not OK to refer to the parents as ancestors and their children as descendants? Or does everyone have to be considered a descendant?
But, yes, you are right. I would like to line of descent to issue from the line of relationship between spouse/partners.