Why should the sibling who was attacked be the one to make nice? Why shouldn't the one who wasn't nice be the one expected to be the one offering apologies.
When a relationship is important, sometimes you have to be the one to make the first effort, even though the other person is at fault. I don't think there was any suggestion that she should apologize for searching, or stop searching, only that if the relationship is important to her she should try to mend the rift.
This could mean saying, "I want you to know that it wasn't my intention to upset you. I don't regret my decision to search, and I'm still going to continue to do what I think is best for me. But our relationship is very important to me, and you ARE my real sister. I'm not going to lie to you, but if it would make it easier, I can agree to not discuss it with you in the future since I know that it's upsetting for you."
Yes, sister should be the one to apologize and make the effort, but if her pride gets in the way that might never happen. Is it fair? nope. But would you rather be right or be happy?