I would tell her you are uncomfortable about it, but since there is a child involved that you will discuss it this one time. It's likely she is going to want to vent, I would shut that down pretty quickly. I would tell her what you know and leave it at that. I would remind her it is b/c there is a child involved and she is obviously a danger to the child and to herself, not to mention the other innocent drivers on the road.
And, I would have the nearest Alanon meeting time and place to give that is near the sister and encourage her to go.