if you convince Anomen to don't take revenge, you get 10K xp plus another 7.5K xp.

But you cannot investigate... the right solution! No murder, investigate, and take murder to the justice!! You cannot do it!
Go for the assassination of an assassin
or nothing!

It's the right solution, or you have to convince him to taking revenge of the
murder?

If he act again his aligment and "knighthood" he can do the quest to become a Knight?

P.S. What i have to do to make "his" Knight Quest!

You get 2 choices:
Revenge or Let Go.

Revenge means he won't get to be a knight else he will become one.

I feel there should have been more to it, like you said investigation. Even if there was no XP gain or other solution, but the fact that all can be settled in one hour without talking to Saerk is a bit too simple.
Maybe you can visit him after he becomes a knight?

IIRC solving the sister murder is a cleric(?) subquest, you talk to the servant at Anoumen's place and find out that it wasn't the prime suspect, it was an 'inside job'

Your Main Charater MUST be a Cleric to do the "sister" quest?

After selecting the "no vengence" option
I visit the "may be" murder palace, all the
guard attack me since Anomen was in my party!

I'll try again when Anomen is a Knight.

Something is sure: the ennemy of Anomen' father is guilty of the murder. In fact, it's the son who killed anomen sister. You learn that when you decide to help anomen to take revenge.

I tried to go back in this house after becoming a knight. There is some guards that attacked us, but nobody else.