First cousin, once removed.
Your husband's third cousin is just considered your third cousin through marriage.
Your husband's cousin's sister is also your husband's cousin. You are not related to your husband's cousin.
Your grandfather's cousin is your cousin twice removed. Her husband has no direct relationship to you.
Let's see if we can unravel this. Your sister's husband's father's brother is your sister's husband's uncle. The uncle's daughter is your sister's husband's cousin. There is no relationship to your sister, unless your sister wants to call her a cousin-in-law. To break it down to its simplest terms: She is your brother-in-law's cousin and no relationship to you.
Only you hold the key to why you had an affair with your husband's cousin. It's called cheating for one.
There is no relationship between the two of you. You do not share a common ancestor. To your husband he would be a second cousin once removed. Most families would refer to them just as a distant cousin.
The relationship between you and your cousin's spouse is that of a cousin-in-law.
Your husband's cousin's grandson is not related to you at all. You do not share a common ancestor.
Regardless of what kind of cousin you are talking about (first, second, fifth, once removed, thrice removed, etc.) your cousin's husband is not related to you. If you need to refer to him other than by name, you can call him "my cousin's husband" or "my uncle's son-in-law."
Your parent would be a first cousin. She would be your husbands first cousin once removed.
You would be a first cousin by marriage, in other words, there is no direct relationship between you.
Your cousin' spouse becomes your cousin by marriage (cousin-in-law). The reason for this is that they have the same relationship to you.