It's good to see that your sister is so understanding and willing to work with you on plans that won't hurt you or her family. It sounds as though you will be able to spend time with your sister and her children after all which is important.
Letters are a great way of purging what you feel - even if you don't end up sending them. You may also consider reading the letter out loud to your T. It is a very powerful experience when you read it and hear the words, especially when heard by someone else in a safe environment.
Finally, as far as your father's calls are concerned: that's what answering machines are for. They are a great way of "screening" when you are not feeling strong enough to deal with the person at the time. When it really is bad, call blocking comes in really handy