You might feel angry in many ways: because the abuse event happened, because it happened to someone you care about, you could be angry towards the abuser or even because you were not able to protect the victim.

 Everyone experiences anger from time to time, but after you find out that a loved one has been abused, it is understandable that you can be angry for quite some time; it becomes problematic when anger becomes expressed through violent or aggressive behaviour. It’s important therefore to distinguish between the feeling of anger and the expression of anger.  

If you know the abuser it could be even harder to manage because maybe at the beginning you won't believe it's true or you would try to minimize the situation finding excuses to understand what happened. 

Experiencing those feelings it's ok as long as you can express them in a safe way. Also remember that the victim needs to feel believed and supported.

Here we share some links if you want to read more about this:
https://www.kcrasac.co.uk/self_help/anger-self-help-guide/
https://www.livingwell.org.au/managing-difficulties/dealing-with-anger/