No it shouldn't be capitalized.
In-law
No, because it isnot a propernoun.
sister-in-law
You only capitalize when you are using their actual names. So if you had two brothers and one sister you would write: "I have three siblings, Marco, Katerina and Jack."
The plural form of sister-in-law is sisters-in-law.
In-law
no!!
There are two ways you can have a sister-in-law. In one case, her sister is also your sister-in-law and in the other she is not. Your sister-in-law can be the sister of your spouse. Then your sister-in-law's sister is either your wife or is also your sister-in-law. Your sister-in-law can also be the wife of your brother. In that case, your sister-in-law's sister is not related to you.
Yes, the sister of your spouse (wife or husband) is your sister-in-law. In addition, the wife of your brother is also your sister-in-law.
Yes; when speaking of a specific law by its short title, you could capitalize it as it appears in the law.
the Compound of Sister In Law is Hyphenated This is the example Ex. Sister-In-Law
You should capitalize "sister" when it is used as a proper noun, such as when directly addressing someone by their role or title ("Hello, Sister.") or when used as part of a religious title ("Sister Mary"). Otherwise, it should be in lowercase.
If your sister in law is your wife's sister then she would be nothing to your sister. However if your sister in law is your brother's wife then she would also be your sister's sister in law.
The possessive form for her sister-in-law is her sister-in-law's.Example: She borrowed her sister-in-law's car.
The possessive form of the noun sister-in-law is sister-in-law's.Example: I borrowed my sister-in-law's car.
Sister-in-law is singular. The plural form is sisters-in-law.
No, your wife's sister-in-law may be your sister. Your wife can also have a sister-in-law who is not related to you; that would be the wife of her brother.