My sister is married. He is my brother-in-law.
I am married, so he is my sister’s brother-in-law.
What are our husbands to each other? Are they also brothers-in-law, even though it’s one extra step of relation? I mean, I know it is in general usage, but we got to wondering today.
Also, has anyone ever heard of a grand-aunt/grand-uncle as opposed to a great one? I guess it’s like grandparents come before great-grandparents?
Unfortunately, english nomenclature for kin is not real specific. We lack words to define those relationships precisely. Some languages have words which specify maternal or paternal uncle, sister’s husband vs. husband’s brother, etc.
I always bemoaned the fact that the terms granduncle/aunt weren’t used more commonly, as it would add some clarity in terms of relationships.
Oh well, given how difficult it is to get people to comprehend the difference between a second cousin and a first cousin once removed, maybe it’s just as well that there aren’t more terms out there to be misused.
Hopefully someone more helpful will be along soon.
Know little about English, so will talk about Thai. 
In English, “brother-in-law” can mean either sister’s husband, or spouse’s brother. In Thai these would be different words. (Actually there are eight words altogether which might translate as brother-in-law or sister-in-law. Four of these words can be applied to the extended relationship, spouse’s sibling-in-law, you ask about.) Funny: Although my Thai is rather fluent, I always have to stop and think about which of the “brother-in-law” words to use.
Thai has a one-syllable word (“Dong”) to express an important relationship expressed only with difficulty in English. If you and I are dong, our children are married to each other, i.e. you are my daughter-in-law’s parent.
Dong is actually how I refer to my brother in lane too, and I don’t speak Thai.
The Spanish for that is consuegro (with the o/a variant for m-or-n/f and -s in plural). Take a word which expresses an in-law relationship: cuñado (brother in law), suegro (father in law)… now add “con” (meaning “with”) in front and you get one step beyond. Concuñados are married to siblings; the children of consuegros are married.
In English there are no specific terms for the relationships you’ve described. My sister’s husband can describe my husband as his sister-in-law’s husband or as his wife’s brother-in-law. In reality they would probably say “brother-in-law” but that is not correct and could certainly be misleading if there were more siblings involved. Fortunately for us, my sister and I both are married to men who have no living siblings, so in our case the difference is immaterial; we all know who’s being talked about.
I don’t agree that it’s incorrect for the guys in the OP to refer to themselves as brothers-in-law. It’s an expansion of the original meaning, but it’s well within the common usage now, and there are few situations where it’s truly important to distinguish between the relationship you have with your spouse’s sibling and the relationship you have with that person’s spouse. (And it’s easy enough to specify “my sister-in-law’s husband” if you need to for some reason.)
–Cliffy
American here. I consider my wife’s sister’s husband, my brother-in-law. And I consider my wife’s brother’s wife, my sister-in-law.
I however do not refer to my brother’s wife’s brother or sister as a relation to myself.
My mother has a lot of siblings (6), who are all married. Since the siblings were inlaws to all the spouses, my father and the other spouses became known as the outlaws. It’s not a real meaning of the word, but it’s cute, and you’re welcome to use it.
Yes, that relationship is included in “brother-in-law.” MY OED is the 1920s edition, and it includes it.
In my experience, genealogy buffs are more likely to use “grand-” than “great-” YMMV.
Another relationship that standard English doesn’t have a term for is the cousins of one’s cousins, on the other side (so there’s no blood connection between you and them). In my extended family, we call those “turkey cousins” (because the largest such group, we say are all a bunch of turkeys), and I’ve been trying to popularize the term.
It is also interesting to note that “cousin” is the only word used to describe a kin relationship without being gender-specific.
(I said “only word”, but I won’t be surprised if some of the other word nerds here come and give me some of the lesser-known kin relationship words that also fit the category.) 
Or at least, the only one for which there is no gender-specific equivalent. Otherwise, “sibling”, “parent”, and so on would also count. While we’re at it, is there any gender-neutral form for “niece/nephew”, or “aunt/uncle”?
“Sibling-once-removed” ??
All four of these are “0’th cousin once removed.”
Thai has four words for aunt/uncle, but it’s always the sex of the older sibling that determines the word! That is, the four words are:
[ul][li] อา = father’s younger sister or father’s younger brother[/li][li] น้า = mother’s younger sister or mother’s younger brother[/li][li] ป้า = father’s older sister or mother’s older sister[/li][li] ลุง = father’s older brother or mother’s older brother[/li][/ul]
While pedantically it’s generally alleged to be limited to sibling’s spouse and spouse’s sibling, both common usage and the logic of relational terminology support its extension to include the reciprocal relationship of spouses of siblings.
In over 60 years of discussing relationships with people from childhood meeting old relatives to the present, until these discussions on the Dope I had never run into the term “grand-aunt” or “-uncle”.
Or it could be because you see them only on Thanksgiving.
How many of those involved were genealogy buffs?
I began using the term when I took up the hobby.