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.
When tracing your family history, however, it can be important to understand the various types of cousin relationships. What is a Second Cousin? The degree of cousin relationship is based on the most recent direct ancestor that two people have in common. First cousins are the people in your family who have two of the same grandparents as you. Second cousins have the same great-grandparents as you, but not the same grandparents.