These terms are usually written with a hyphen: mother-in-law and father-in-law. They mean the mother and father of your husband or wife. If your spouse has brothers and sisters, they are your brothers- and sisters-in-law. If you have children and they marry, you will have sons- or daughters-in-law. In theory you could use the term for any relationship - cousins, aunts etc - but in practice people mostly just use the terms above.
It just means that the person has a relationship with you, which is through law (marriage) instead of blood. Originally "in-law" was also applied to step-families - a mother-in-law might be a stepmother as well as a spouse's mother - but we no longer do this.
It just means that the person has a relationship with you, which is through law (marriage) instead of blood. Originally "in-law" was also applied to step-families - a mother-in-law might be a stepmother as well as a spouse's mother - but we no longer do this.
