Sass mixin for styling elements based on sibling count.

siblings-by-count.scss

// -----------------------------------------------------------------------------
// @author Zakir Tariverdiev
// @created August 2, 2014
// @desc Sass mixin that provides convenient way to style element's siblings. 
// Based on the example from Lea Verou's blog:
// http://lea.verou.me/2011/01/styling-children-based-on-their-number-with-css3/
// @param (string) $sibling-count - number of expected siblings. You can also use
// use such forms as "n + 3" and "3n" for "at least 3 siblings" and 
// "siblings divisible by 3," for example. These forms must be enclosed in 
// quotes for proer parsing though.
// @param (string) $sibling - sibling element you wish to target. In most cases
// this parameter will need to be enclosed inside quotes for proper parsing.
// @param (string/optional) $every-nth - With this parameter, you can specify 
// that only a specific sibling will have the specified styles applied to it.
// Ex. "3n + 1" will have the 1st and every 3rd sibling use the styles.
// Checkout example of this mixin in action here: http://codepen.io/zakirt/pen/wClId
// -----------------------------------------------------------------------------
@mixin sibglings-by-count($sibling-count, $sibling, $every-nth: null) {
  $colon: ":";  // Can't use literal : in expressions, so we'll use this
  
  @if ($every-nth) {
    $sibling: #{$sibling}#{$colon}nth-child(#{$every-nth});
  }
  
  &:first-child:nth-last-child(#{$sibling-count}),
  &:first-child:nth-last-child(#{$sibling-count}) ~ #{$sibling} {
    @content;
  }
}

Been looking for this for a while! Have you ever written stylus? Any chance you could convert for it?