Lucius did not stop Narcissa.
This was not something he could stop. It was obvious that Narcissa was furious right now, and anyone who tried to block her at a time like this... well, Lucius figured he might end up sleeping in his study for a very long time.
As for the other Pure-blood families, they did not know Elwin's true background. But just for the fact that Elwin was about to become the King of Slytherin, they would take action.
Chapter 133