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.
First cousins are two children who share a set of grandparents, but have different parents. Double first cousins share both sets of grandparents, but have different parents. Let's state that in a different way: double first cousins happen when two siblings of one family marry two siblings of another family. When you and your brother marry Bob and his sister, your children and their children will be double first cousins. Many states in the USA allow first cousins to marry *; but as far as I can find only one, North Carolina, will allow cousins to marry but will not allow double first cousins to marry.