I admit that I have reservations that a family member may feel pressured by their sister (or daughter) to allow her time alone with our daughter. My x WILL put up extraordinary pressure and our daughter will also do so. So I expect a high likelihood that despite sincere promises, the visitation would not end up supervised.
Of the three main options (x's sister, brother and father), my first choice is the brother for a couple of reasons. He is the only one out of walking distance from x, about a 20-minute drive. He has a wife to help supervise and, best of all, he is a cop who is used to dealing with addicts and maintaing order and rules. He will not allow cigarettes, pot or beer to be given to his neice.
So what if the supervision could be correctly enforced? What harm is there in our daughter staying with her uncle and his wife, who is a very nice lady? As long as strict rules were in place for my x to visit only in their presence, what kinds of things could go wrong?
On the flip side, isn't there a harm in going through teen years without seeing one's mother? Yes, x could come back to Florida as she did this past Christmas, but keeping them from being alone together is high drama and high conflict. The two of us together, with or without an attempt to abscond, is a powder keg of tension and conflict at all times. What kind of parenting time is that? How does this benefit our daughter?
I appreciate everyone's concerns and comments. It does give me plenty to think about. These are not easy issues.