This is a sample apology letter to your sister.
I am thankful to God for giving me such a lovely and wonderful sister who is very considerate, sensible and always caring for me. You are so close to me that you know almost all my secrets. We speak almost daily on telephone and share everything that is happening in our lives. You are always guiding me and thus making a good person out of me.
I have no hitch to accept my mistake, committed on the other day and I beg apology from you, my dear sister, for my absurd behavior in the party on that day. I am unable to understand as to what came upon me that I screamed at you for no concrete reason. I am extremely sorry for my misbehavior.
I have realized now that sometimes and on occasions when I see you vulnerable, I start behaving as Dad. When I do that, my expectations from you become unreasonable and I become extremely emotional and irrational. I have actually realized my mistake. I will try my best not to behave like Dad in future.
I really cherish the special connection we share, unlike many siblings in this world and I'll do anything to keep it that way.
Your loving brother