There are two ways your father's only brother can have a sister-in-law.
The sister-in-law of your father's only brother could be your mother. If, instead, she is the sister of your uncle's wife, she is not related to you at all. You share no common ancestor with her.
Obviously not...Unless your brother is going to be a d-bag to your boyfriend :)
very hard because you would have very limited contact
Yes, niece and nephew-in-law are valid terms.
Since close is a relative term, it's debatable. They are in the same hemisphere. However, I would say no, as their closest points are about 7,800 miles apart.
It means that when you have a friendship, and you two are VERY close people would (or the other person) declare you his as if you were a brother, or sister to him (vice-verse [sister declare you as her brother, or sister]).
people did pretty weird things back then like marrying their brother, sister, cousin, aunt, uncle, or close relative. (excuse my not using capslock). in this instance she was forced to do this to ensure that there would be pure royal lines. This was a bad idea...marrying your brother is sick, first of all, and second of all, the genes will be severly mutilated. Things like this cause diseases and retardation.
Well they are brother and sister! Who would ask such a dumb question. That would insult bot Leland and Lyssa. Whoever you are you think way wrong. I have a brother and love him very much so in that way your sort of insulting everyone who has brother. So anyway they are just brother and sister and are very close but not in that disgusting way!
the original dynasty would die out in battle or of disease, then the family of a close relative took over
That depends on your relationship with them, in most cases they would not be considered a close relative as the nearest common ancestor is 5 generations away. Third cousins can very well become husband and wife. In that respect they would become very close, but they are not concidered closely related from a genetic point of view.
You have no close family relationship with him, in the western European tradition at least. So you call him your brother-in-law's brother. Other cultures may vary.
Yes, if your close relative is living in the home with your permission, they can be considered a tenant even if they are not paying rent. This means they would have certain legal rights as a tenant, and you would have certain responsibilities as a landlord. It's important to have a clear agreement in place to define the terms of their occupancy.