Its a tough position since it's a sibling. I can only speak from my experience, but the steps tell us to make amends wherever possible except when to do so would injure ourselves or others. To me, this is one of those situations where the amends would cause more harm than good because of the circumstances and my sponsor would have told me to stay the hell away, because bringing up past things like this could cause more emotional damage to the victim.
I don't know the family dynamic, but seems to me like this creates an elephant in the room type situation, so the amends is warranted.
In going through the amends, I've always been told to make them in person, say my part, then shut up and listen to what the other person has to say. Seems to me that that opportunity wasn't given to your loved one. The whole purpose of the amends is to set things right with the harmed person and honor their wishes. I think a response letter for them to say their part is more than warranted.