Here's the way I see it Blue:
There's two types of people in this world, the kind that tell you what you want to hear and the kind that tell you what you need to hear.
Your parents, your siblings, your GOOD friends and the people who generally care about you and take your best interests to heart are the people that tell you what you need to hear. The other 95% of the world you meet really don;t care about you or what happens to you. They want to tell you what you want to hear so everything is happy and you can continue to live inside your safe bubble.
You know from personal experience what it means to have people tell you what you need to hear. Usually you feel hostile towards what they say and generally don't want to hear it, but in the end they are always right because they care about YOU.
It's a choice only you can really make as to whether you want to step in and weather the storm for your sister, but you know how much you wish you would have heard what people thought before your ex. It takes a certain caliber of person to step up and say what everyone else is afraid to say.