Uncle is apparently 'wuj' or 'wujek' - which one of these is more commonly used? Is there any difference between the two?
same as between father and dad - ojciec i tata. Wujek definitely more common and wuj is more formal
ciotka and ciocia = same difference as between wuj and wujek
You can address stryjenka and wujenka as ciocia - very common and acceptable