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.
Generally a cousin who is not a first cousin is usually referred to as a distant cousin. However, I know people who refer to all of their cousins, first, first once removed, 2nd, 3rd etc as just cousins. So when a person ask a question about a cousin I wish they would distinguish the specific relationship.