Yes it can in some instances help to clear the bladder.,the problem is that it is difficult to get cranberry juice that has no added sugar or sweeteners.
A health food shop may have a pure juice,bu this may need to be diluted. I have not tried the juice on a piggy myself as I have not had a case of cystitis for 3 years(famous last words, I will probably find all my sows sqeeking as they pee now!) but I have read on the forum that other members have had sone success with this.Worth a try anyway.
What are the piggies symptoms,and is it a boar or sow>?
Any pig with suspected UTI needs lots of extra water anyway,give this with a syringe.If you do try the cranberry,give that by syringe as well,do not put it in the drinking water .