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.
I refer to my mom's cousins as my cousins, and always refer to them as Cousin Ralph and Cousin George and I am always Cousin Jess.. So I call my first cousin's and my mom's cousins cousin.. We find it to be a bonding thing. If you don't call Jane Cousin Jane then Dear Jane is just fine.