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.
Children of your parents' brothers and sisters. You and your first cousins share one set of grandparents. Double first cousins. If a pair of brothers marries a pair of sisters, their kids are not only first cousins, they're double first cousins: They have both sets of grandparents in common.