I wonder if he would agree with you that he was "strong." I don't think that losing people close to me has made me any stronger, and none of them were murdered. He is surviving and moving on. I have no idea if his offer of forgiveness was real or wishful thinking or something else.
Some things are just about impossible to really forgive. You might say the words, but if the wound is deep enough, they mean nothing. Your body knows better.
The fact that he hasn’t killed himself indicates to me he has some measure of strength. Telling the man that raped and murdered your sister that you forgive him, to his face, no less, is strength. He’s managed to make his life work. Many people decide to give up on life because one person they know dies. He’s had to deal with 6 or more. It’s hard for many people to see their strength, and sometimes it’s easier for others to do so. From what I can see, this man has strength.