Law Abiding Citizens.
I would call them spectators, sitting in the public area of a law court.
There is no such term as handle on the law. However, there is a radio station call in show called Handel on the law where people call in for legal advice and discuss other types of laws.
Your husband's Brother would be your Brother-in-law. Some people feel that his wife would be your Sister-in-law, other people feel his wife should be called "My Brother-in-law's Wife". I prefer to call her my Sister-in-law. Perhaps you could ask her what she'd prefer to be called.
Perhaps a law stating that all parking spaces must be rectangular.
The English language has no term for a relationship between you and your husband's sister's husband. Most people would not consider her to be related to you. You could call her your sister-in-law's husband.
Your mother-in-law. Or, call her by her name.
The relationship of the sibling of your sister-in-law or brother-in-law to you does not have a name in English. You could call them your brother- or sister-in-law if you want, or you can call them the brother or sister of your brother- or sister-in-law.
You still call her your mother in law or if you were divorced you would call her your Ex- mother in law.
There is no actual blood relationship. In most instances you call them the brother of your daughter in-law. Or just introduce them by name and let people guess if there is a connection.
I would say a peacefull activist
A common law marriage