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.
Your parent's first cousin is your first cousin once removed. The child of your first cousin is also your first cousin once removed: your grandparent is that child's great-grandparent. You can do the whole removed thing for every category of cousins -- second cousin once removed, and so on.