Question:
Is it yaharog ve'al ya'avor to kiss one's four year old niece because of abizrayhu?
Answer:
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.
Sources:
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.