Let us focus on the second question first. According to the philosopher, Trudy Govier, there are distinctions among primary forgiving (in which you were directly hurt by another), secondary forgiving (in which you are resentful because of injustice toward another person about whom you care), and tertiary forgiving (in which you are resentful toward someone who is quite distant from you or a loved one, such as a politician who behaves badly). You are discussing secondary forgiving because you are resentful of another who behaved badly toward your mother. So, yes, you can legitimately work on forgiving this person.
Why is this one so hard? I think it is because your mother likely is going through much pain because of the person’s offense and you are reacting to this deep pain in your mother. Secondary forgiving is not necessarily always more difficult than primary forgiveness. The difficulty depends on the depth of the injustice and the depth of hurt experienced by your loved one and you.
For additional information, see Can You Forgive an Entire Group?