Do you speak still French at home? In other words has your daughter been able to keep up in the language. If French is not used very much, then you might want to start using it about a month before your relatives get here. Jdtm is very correct when he mentions that kids with two languages will be "silent". For that matter 5 year olds can be very silent around unknown people.
However, I think the main change has to do with the relatives. Instead of calling for your daughter to be disciplined and forced to interact with them , they should be going out of their way to get her to want to talk with them. Lord, they sound scary to me, what must they appear like to a kid? They need to try a little sweet talk (maybe in English) and accompany that with some sweets.
One internet site I might suggest is "selectivemutism.org" - start by reading the FAQ's to see if the descriptions are similar to those of your daughter. If so, there is lots and lots of help on that site.
However, it is not unusual for children with two languages to be "silent" in the less familar one for a period of time until they become more comfortable in that environment. But, if your daughter has been silent for her entire life with these relatives (and I am assuming that she sees them often), then you might have to consider that your daughter might be selectively mute. By the way, if selective mutism is the issue, this type of anxiety is highly treatable. I wish you the best ....