Per a family member who is more familar with things military during this period than I, it would depend upon the year in which S & S is actually set as to whether Brandon's sister would likely be in France. Starting in, or shortly after, 1803, Napoleon rounded up British citizens and either sent them home or put them in jail. This caused great outcry in England, as this had not been the norm up to that time. However, some British citizens were given passes to go to France for conferences, etc., but these would be more notable folks than Brandon's sister: scientists, etc. Also, unlike wars in 20th century Europe, travel would not be prohibted in countries that were non-participants in the conflict.