There have been discussions of incest here before. Since we don't know how old either of the people are or where they live - whether or not it is incest LEGALLY is unknown.
Here are a few definitions:
England and Canada prohibit marriages between uncles and nieces and aunts and nephews but allow first cousins to marry. Some U.S. states follow he same pattern, but others have made first cousin marriage illegal. ( Because these are considered incestual relationships.)
and
A. Sexual contact, inappropriate touching, or sexual penetration involving people who are closely related -- for example, a father and daughter, sex between a brother and sister, sexual contact between a child and his uncle. Sex between persons of unequal power can also be considered a form of incest, such as between a therapist and patient, or a student and teacher
B. There is now an evolving definition of incest that takes into consideration the betrayal of trust and the power imbalance in these one-sided relationships. One such definition is: "the imposition of sexually inappropriate acts, or acts with sexual overtones ... by one or more persons who derive authority through ongoing emotional bonding with that child." (Blume, 1990, p. 4) This definition expands the traditional definition of incest to include sexual abuse by anyone who has authority or power over the child. This definition of incest includes as perpetrators: immediate/extended family members, babysitters, school teachers, scout masters, priests/ministers, etc.
C. "Incest between an adult and a related child or adolescent is now recognized as the most prevalent form of child sexual abuse and as one with great potential for damage to the child". (Courtois, 1988, p. 12)
D . The definition of incest varies from one group to another. In the United States, for example, marriage between first cousins is illegal in some states, but legal in others. Americans don't carry the ban against marriage beyond first cousins, but some groups carry it much farther.
E. The Arunta, a tribe in Australia, for example, look at relationships in an entirely different way than we do. They think of certain clans as being "blood" relatives, and marriage between people in those clans as incest. They reckon such blood relationships so extensively that for Arunta men marriage to seven out of every eight women is defined as incest.
F. Perhaps one of the most trying issues for the worker is that of sibling-to- sibling sexual abuse. In this article, when sibling incest is mentioned it is referring generally to older brother abuse of a younger sister. Recognizing the risk factors, understanding the causes, and making an assessment, can be challenging.