No.
Yes, it is legal for your sister to marry your husband's brother.
Since your sister's husband's daughter is usually your sister's daughter, you are her aunt or uncle. If the husband's daughter is from a prior marriage (before he married your sister) then the daughter is not related to you. However, if the daughter is young, her father may have her call you and treat you as an aunt or uncle.
Your sister's husband is your brother-in-law. The daughter of his sister is his niece, and your sister's niece, but is not related to you. Assuming the child was born to the woman before she met your brother, the daughter of the woman whose brother married your sister is not related to you.
Not if he is still married to your sister.
Your sister-in-law's husband is never your brother-in-law. He can be your brother, when your sister-in-law has that relationship to you because she is married to your brother. You could also have a sister-in-law who is the sister of your spouse. But then her husband is not related to you.
Since your sister married a woman then her mate would be your husbands sister-in-law. However, if the woman your sister met dresses and acts like a man and your sister's mate requests it then she would be your husband's brother-in-law.
Someone who stole her half sister's husband?
Erebus was the consort of Nyx, not husband.
If Jane and John are married, and Jane has a sister, Jane's sister is John's sister-in-law. Jane's sister's husband, however, is not related to John, although he is ''Jane's'' brother-in-law.
I imagine you are talking about your spouse's sibling's spouse (your wife's sister's husband). No.
My husband's sister-in-law or my brother-in-law's wife. She is in no legal relation to you.Most people would just refer to her as your SIL as well, but technically she's just your husband's sister-in-law or your brother-in-law's wife.
As long as there is not a vast age difference and they are dating then no, there is nothing wrong with your sister being with your husband's brother as long as neither of them are married to someone else.