I don't think I'm going to tell my partner about her brother assaulting my friend.
I really don't think that's a great idea.  Why would you keep this assault from her? What about your friend that was assaulted?
The whole thing was terrible, and I don't want to talk about it with her.
Firstly, this is something that's too big to keep secret. Secondly, I understand talking about it is unpleasant but she will hear about it eventually, so better she hear it from you than someone else secondhand.
Okay, that makes sense, but I need to process it first.
I don't think you should wait too long before saying anything, that's how it will catch up to you.  We shouldn't keep things back from our loved ones.