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.
Cousin (a.k.a first cousin): Your first cousins are the people in your family who have two of the same grandparents as you. In other words, they are the children of your aunts and uncles. Second Cousin: Your second cousins are the people in your family who have the same great-grandparents as you., but not the same grandparents.