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.
There's a good chart on Wikipedia explaining the cousin thing... then baby is your 2nd cousin once removed. If you have a child then your child and the baby will be 3rd cousins. Your cousins' daughter is your first cousin once removed so HER baby is your first cousin twice removed. Figures to do with removed refer to generations, first/second/third refer to degrees of separation so first cousin means your parents are brothers or sisters, second cousin your parents are cousins and so on.