As a fellow abused child I hail your decision
You did the right thing when no one else would, is your sister a victim also, if not maybe one rationalize her position as in her mind it never happened. What I don't undertsand is why she would think her daughter is not in danger.
I am so proud of you for standing up to them, you may have saved a young lady from years of pain, you are a hero in my book.
Please stand strong, I wish I had outed my cousin years ago, I have no idea how many people have been hurt by him.
If you need someone to talk to I will be here,
Best of luck to you.
Bill