I don't understand why a child won't, or shouldn't, get emotionally attached to a man who is their mum's dp/dh and parent to their half-sibling?
If my marriage had broken down, and I had formed a new relationship, I would want my new partner to treat my children from the previous relationship in a kind and caring fashion. If we went on to have children together, I would want him to treat all of the children equally, and I would hope that he had a loving, parental relationship with all of them.
I don't think I would be able to carry on a relationship with a man who didn't care for my children from the previous relationship - I cannot see how it would be good for them to see me choose to stay with someone who didn't treat them kindly.
The alternative would be him treating my children differently, and less well to the way he treated our child/ren - and who would think that was a good thing?
And if my dp/dh was treating my children in a loving, kind way, and developing a parent-type relationship with them, surely they would, over time, become attached to him?
IMO, family is not just biology - it's about loving, caring, nurturing relationships. I am not related to dh by blood - but no-one batted an eyelid when we got married and called ourselves a family. Equally, no-one bats an eyelid when people adopt a child and the child calls them mum and dad, and no-one would think it at all strange if the child became attached to their adoptive parents.
If I had a blended family, I would want it to be as strong a family as it could possibly be - and I don't think that would be possible if my new partner didn't at least try to have as good a relationship with my children as he did with our children.