I wouldn't mind if my girls went onto their siblings from my ex, but we have a very different relationship where all four adults not only get along but like each other and spend lots of time all as one unit!
We are blessed to have that relationship, but blood is blood. Perhaps it's only a piece of paper but I really feel it adds a divide that is not needed.
Like I said, I feel sad about it, and others won't depending on circumstances.

