I don't think that there are any "right" or "wrong" questions, or any questions that "should" be asked. If you are going to the meeting as an ambassador for the family, then I would recommend finding out from other family members if there are any questions that they want answered. Other than that, just ask any questions that you have. If you can't think of any questions, don't worry about it. Some might come to you when you are there, and I'm sure that you could call later if any came up.
Have you considered taking another family member along (if it is possible)? You might also take a notebook to write things down that you want to pass along to your family. This must be a very difficult time, but remember that the person you are meeting with understands this and has experience dealing with these types of situations. I wish you the best.