How to convince my sister not to move to a VERY bad part of the city for her boyfriend?

My sister's a moron! She left her husband of almost eleven years just last year over absolutely stupid reasons, and is now planning on moving in with this guy she barely knows. Even worse she's taking her two children along for the ride. But my real gripe is that this guy lives in the WORST part of the city. Half the houses on his street have boarded up windows. The rest should be condemned. He openly jokes about his neighbor's selling drugs. There's regularly shootings and assaults in the area. Plus the school district is just plain awful. The children would be going from a rural school to a school where they'd have to walk through a metal detector, and be stuffed into over-packed classes. Everything about this idea makes it a horrible decision, but my sister's "in love" and won't listen. She's acting like a moron. Her ex is pulling his hair out over this because he's afraid of his children being beaten or worse, and I feel the same way. How can I get this woman to stop acting like an absolute moron, and start thinking of herself and the children?