-cest +

A romantic or sexual relationship with someone who lives in the same co-ed residence hall at a college or university.

Sexual intercourse with one's brother.

1. Sexual intercourse between people who are too closely related to marry (as between a parent and a child).
2. Sexual relations between people who are so closely related that their marriage is illegal or forbidden by custom.

The statutory crime of sexual relations with such a near relative.

3. Etymology: from about 1225, "the crime of sexual intercourse between near kindred", from Latin incestum, "unchastity, not chaste, impure"; also specifically, "sex between close relatives"; from in-, "not" + castus, "pure".

In early use also with reference to "sexual relations" or "marriage with someone under a vow of chastity; sometimes distinguished as spiritual incest".

1. Relating to or involving a sexual relationship between two people who are considered, for moral or genetic reasons, too closely related to have such a relationship.
2. Having had a sexual relationship with someone who is considered to be too close as a relative.
3. Unhealthily intimate or interconnected; especially, so as to exclude the involvement or influence of others; an incestuous friendship.
4. Being so close or intimate as to prevent proper functioning: "There was an incestuous relationship between organized crime and government."

A mother who has sexual intercourse with her son.

1. Having sex with one’s father.
2. Sexual intercourse with one's daughter.

Sexual intercourse with one's sister.