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.
You and the children of your parents' cousins are second cousins and share at least one great-grandparent. Your child and your cousin's child are second cousins. Third cousins. You and the children of your parents' second cousins are third cousins and share at least one great-great-grandparent. And so on with the fourth, fifth, and sixth cousins. First cousin once removed. A relationship that is removed is one that exists in two different genealogical generations.