I think the attack was set up by the Kaisel family so that the prince Ayle/Ail start trusting his kinght commander Luce/Ruth Kaisel. If the family really wanted to kill Aule, this set up was too obvious, and they wouldn't have let Luce get in the way. Not because they want to protect Luce, but to prevent him saving the prince, because even though Luce didn't earn his new position by his own merit, he still has some skills as a knight.
Later on, when the Kaisel family will have made cynical Ayle lower his guard in front of sweet, earnest Luce, I think the family will threaten Luce's mom's life so that Luce kill the prince, or they are going to use Luce without him knowing to kill (probably with poison) Ayle.