The most commonly used word would be incest, although technically, incest refers to sex between (close) relatives. However, since a marriage is usually supposed to be consummated (meaning the partners in the marriage do have sex), the word is also applied to marriage.

What do you call it when siblings date?

When sexual relations occur between siblings, its called incest.

What do you call sisters boyfriend?

A brother-in-law (plural brothers-in-law) is the brother of ones spouse, the husband of ones sibling, the husband of ones spouses sibling [relevant in the first case you mention], or the brother of ones siblings spouse. You can work out what sister-in-law covers.

Can you legally date your sister?

Romantic and sexual relationships between siblings or other family members is called incest, and if one of the people involved are under the age of consent then it is illegal because its illegal for an adult to date a minor.

What is husbands brothers wife called?

Ones husbands brothers wife; or more generally ones spouses sister-in-law, the wife of a brother in relation to the spouses of his siblings. Ones brother-in-law or sister-in-laws sister; that is, ones siblings spouses sister (ones sisters husbands sister or ones brothers wifes sister).

What do you call your sisters husbands sister?

A sibling-in-law is the spouse of your sibling, or the sibling of your spouse, or the person who is married to the sibling of your spouse. More commonly a sibling-in-law is referred to as a brother-in-law for a male sibling-in-law, and a sister-in-law for a female one.