in law, in-law

In law or in law ( two words ) [ in ] the body of rules or regulations written or enacted by a government ( law ) --
In law, the king is head of their government, but the generals have all of the power.
He learned to argue like that in law school.

In-law or in-law ( one hyphenated word ) means a relation by marriage, usually combined with a term for the specific relation --
He's looking for a gift for his mother-in-law.
She went shopping with her sister-in-law.

Now that you know that, you can say --
"In  law, his mother-in-law will have custody of the children, but they will live with him."

Note -- "inlaw" ( one word ) is an archaic ( no longer in use ) legal term, meaning to restore an outlaw to the protection of the law --
The magistrate agreed to inlaw him, because of the service he had rendered to the duke.