Dear Mary
I'm really happy that you writed me. I missed our discussions. I want to know how are you doing and what is going on in your life, and I also have a lot to say you what happened with me!
But let's think about you sister. I understand you, I have brother/sister and it's absolutely okay if we arguing sometimes. It's always happen between family members. But you don't have to be upset about this.
You and your sister just need to calm down and think about this, analyze everything. Was it your or her fault, may you both weren't polite. After this have a talk
Good luck, I hope my advice can help you
Your friend,