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.
The child of your 1st cousin and the child of your great aunt or uncle are both your first cousin once removed. Note that the number of generations difference is the same: The relationship from a child of your great aunt or great uncle to you is the same as the relationship from you to a child of your 1st cousin.