In what cases marriage is not allowed?

The following cases are established by the foundations of marriage and family law in which marriage is not permitted.
Firstly, between persons of whom at least one is already in another marriage.
Secondly, between relatives in a straight ascending and descending line.
Thirdly, between full (two common parents) and half (one common parent) brothers and sisters.
Fourth, between adoptive parents and adopted children.
Fifthly, between persons of whom at least one was declared legally incompetent by the court because of mental illness or dementia.

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