Is it yaharog ve’al ya’avor to kiss one’s four year old niece because of abizrayhu?
While one should not kiss his four year old niece, it is not in the category of yaharog ve’al ya’avor. The reason is because a niece is not considered ervah to a person, since a man may marry his niece. In addition, since she is also young we will not assume that she is a nida, therefore the issur to kiss her would be d’rabonon in nature, and there is no yaharog ve’al ya’avor on abizirayhu which is d’raonon in nature.
Shach Y:D 157-10, Igros Moshe Y:D 2 137, also E:H 4-60, Avnei Yoshpe 2-89(4), M:B 75-17.