I would hope that between the time in high school and college these people have matured. Whatever the purpose or reason for this beef should take a backseat to your new experiences with this person as a sister instead of an enemy. While there may have been things that have caused beef between you and her, the new reality is that she is a sister to you. That old stuff should be laid aside. Why would anyone want to stay mad at someone their entire life? There's too much to do in the world than hold a grudge. Actually, I'd let it go regardless of the situation. I consider myself too old (a mere 20 in July) to carry on like a 16 year old in high school
. Now if she wants to continue it, that's a different situation but I would just kill her with kindness.