Mrs. Weasley (maiden name Molly Prewett) wasn't even in the Order of the Phoenix last time around, but her brothers Fabian and Gideon Prewett are mentioned as being on Moody's picture and having died in a fight against overwhelming odds. So her fear is not merely based on what might happen - it's also based on what already happened in her past. Last time around she lost her two brothers; now she has no less than seven children and her husband who might be in danger, including several children who have a propensity for taking risks or getting in dangerous situation, and several other children who are full members of the Order.