Set div to have its siblings width

Answer 1

2020 keep it simple...

Use grid and the fractional unit? Then you can split up into as many equally sized rows or columns as you want:

.container {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  grid-column-gap: 1em;
}

.container > div {
  border: 1px solid black;
  padding: 0.5em;
}

  
I'm a part of a grid. I will be split up into equal parts with my other sibling(s) depending on how many columns the grid is given.
  
I am a sibling element.