I am not a local kiter at Nilsiä/Tahko but I can say that it is fairly forested area which might lead to gusty conditions. But luckily there is a lake at Nilsiä and on January it should have thick ice. It could be frozen already. If you are not 100% stuck to Nilsiä then you can try other lakes (Koli ski resort next to Lake Pielinen, lakes at Kuopio etc) as well. Just check maps.google.com.
About winds - everything is possible. A forecast and current conditions (close-by) can be seen at www.fmi.fi, select "In English" and local weather for "Kuopio" for weather (around 80km away from Nilsiä).
In Finnish only if the link works - http://www.hs.fi/saa/gclid=CI64ncbPuaUCFRgz3godzDApZw
I think on your arrival you can also try to ask about conditions/sites/help in English at local chat http://www.purjelautaliitto.fi/kite/keskustelu/