Like cousin marriages, uncles marrying their neices is also commonly practiced.
Various dictionaries define incest as sexual activity between "close" relatives. Ultimately it comes to down to what "close" is. For some father-daughter is close while cousins or uncle-neice is not.
Some research has also been done which proves that offsprings resulting from marriage/sexual relationship between blood relatives can have genetic abnormalities.
Ewww.
Marrying uncles/aunties and neices/nephews is wrong.
I'm aware that it happens in India.
It can hardly be compared to cousin marriages.