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 first cousin's children are your first cousins once removed. Their children are your first cousins twice removed. It goes both ways, unlike father/son or aunt/niece. They are your 1C1R and you are theirs. If you had a second cousin, his/her children would be your 2C1R and so on.