An acronym is a word or name formed as an abbreviation from the initial components in a phrase or a word, usually individual letters (as in NATO or laser) and sometimes syllables (as in Benelux). There are no universal standards of the multiple names for such abbreviations and of their orthographic styling.
Son and daughter of your siblings and siblings-in-law. As with uncle and aunt, they're not your nieces and nephews by marriage, they're your nieces and nephews, plain and simple, even if they're your spouse's siblings' kids and don't have a single drop of blood in common with you.
If you can't keep your third cousins and your first cousins twice removed straight, you are not alone. But there's a simple way to figure out the relationships between relations. First cousins share a grandparent, second cousins share a great-grandparent, third cousins share a great-great-grandparent, and so on. The degree of cousinhood (first, second, etc.) denotes the number of generations between two cousins and their nearest common ancestor.