Give yourself permission to cry.
Look at your sisterfriend breakup like any other major loss. Let yourself grieve. Express your wounded feelings to trusted family and friends. Many of us keep our negative feelings to ourselves. Not dealing with them means they never really go away.
Talk the talk.
Not with her—but with yourself. If you don’t keep a journal, now is the time to start. Write down your feelings and unexpressed thoughts—the ones the breakup left you with and the ones you never told her about when you were friends. Or write her a letter—one you don’t have to send. Describe the wonderful memories you have of her, ask for forgiveness for specific things you may have done wrong, and offer her your forgiveness. Or say nothing at all. If you can’t say anything good, then at least resist the temptation to bad-mouth her. It only keeps negative energy circulating.