A brother-in-law (plural brothers-in-law) is one's spouse's brother, one's sibling's husband, one's spouse's sibling's husband [important in the first situation you describe], or one's sibling's spouse's brother. You can figure out what your sister-in-law is covered for. Same idea as a brother-in-law, only female.
As for your husband's siblings, they are the other spouses of his brothers or sisters. In other words, they're his wife/husband's siblings. His sisters-in-law are his wife's siblings' wives or husbands, and his brothers-in-law are his wife's siblings' spouses or partners. Not all couples have the same number of siblings, so there may be some omissions from this list.
His parents' siblings' spouses or partners are called nephews/nephews, aunt/aunt, respectively.
Treat them like any other couple - especially his mother and father!
If she's your best friend, he's probably your favorite person too. So treat him like you'd treat her - with respect and affection.
Men love it when you call them by their pet names. Use them often, it will mean even more when they get a surprise visit from you down the road.
A sibling-in-law is the spouse of your sibling, the sibling of your spouse, or the person married to your spouse's sibling. A sibling-in-law is more frequently referred to as a brother-in-law for a male sibling-in-law and a sister-in-law for a female sibling-in-law.
They are also called collateral relatives. This term refers to all of the spouses and children of one's siblings. Thus, a first cousin is another word for sibling-in-law; a second cousin is another word for spouse-in-law.
In some cultures, such as in India, Pakistan, and Bangladesh, it is common for brothers and sisters to marry each other. Because these people are members of the same family, they are called brother-in-law and sister-in-law.
In other cultures, such as in North America, this type of relationship is not considered normal and so they are called brother-in-law or sister-in-law instead. However, in general usage, both terms mean the same thing.
Furthermore, in some cultures where consanguinity (marriage within blood relations) is common, such as in Saudi Arabia, Oman, United Arab Emirates, and Iran, brothers and sisters can also marry each other. In such cases, too, the words brother-in-law and sister-in-law are used interchangeably to refer to this relationship.
Sibling-in-law A sibling-in-law is the spouse of your sibling, the sibling of your spouse, or the person married to your spouse's sibling.
In addition, there are other relative groups that can be formed: parent-in-law, child-in-law, uncle-in-law, aunt-in-law, and nieces/nephews. The terms cousin-in-law and in-law are also commonly used, but they are descriptive rather than familial relationships based on blood ties. Thus, a cousin-in-law is simply someone who married into your cousin group, while an in-law refers to anyone who is married to your sibling or their sibling.
A wife's brother is her husband's sibling. If the wife has two brothers, they are both wives' brothers. Similarly, a husband's sister is his wife's sibling. If the husband has two sisters, they are both husbands' sisters. There are several ways to refer to these relatives, depending on the gender of the person being described: women's brother, men's brother, wife's brother, husband's brother, wife's sister, men's sister, husband's sister.
Your spouse's full, half, or step siblings, as well as your siblings' spouses, are all your brothers and sisters-in-law. As a result, your brother-in-brother law's (i.e., your sister's spouse) is simply your brother-in-brother. Law's wife is called your sister-in-law.
Full siblings are those who share the same parents. If your spouse has a brother or sister who is also your sibling, then they are full siblings. Half siblings have different mothers or fathers, but they can still be friends and play together. When there is no father involved, as in the case of male babies, they are always full siblings. Step siblings are on their spouse's side of the family; they may have the same mother or father, or they could have different families. They can be friends or not, depending on how they feel about each other. If you don't get along with your spouse's stepbrother or stepsister, that doesn't mean that they aren't allowed to visit anymore. They are still considered part of your family.
As for cousins, since they're not your spouse's siblings or offspring, their relationships are a little more complicated. A cousin is someone who is related to you by blood or marriage. In other words, your spouse's first cousin would be another cousin on their mother's side or father's side. Second cousins would be siblings of either your spouse or their parent.