Flexbox Accordion

Question

I'm using flexbox for the first time and I found one very pretty accordion which I need, but width of the inactive boxes makes me a big difficulties.

I have 5 boxes and ratio is 5:1:1:1:1, but inactive boxes are still too big. I want ratio like 15:1:1:1:1 or something like that. But if I increase width of active box, nothing gonna change. I repeat, i'm beginner in flexbox and, how much I understand it, when I increace ratio on active box, width of inactive boxes should be smaller.

Where I'm wrong?

* {
  box-sizing: border-box;
/*   border: 1px solid black; */
}

body {
  font-family: 'Lato';
   -webkit-font-smoothing: antialiased;
    -moz-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
    text-rendering: optimizelegibility;
}

ul {
  display: flex;
  min-height: 100vh;
  max-height: 100vh;
  margin: 0;
  padding: 0;
  overflow: hidden;
  list-style-type: none;
}

li {
  flex: 1;
  display: flex;
  align-items: stretch;
  padding: 20px;
  background: #E74C3C;
  cursor: pointer;
  transition: all .5s ease;

  &:hover {
    background: darken(#E74C3C, 3);
  }

  &.active {
    flex: 5;
    background: #fff;
    cursor: default;

    h2 {
      color: #E74C3C;
    }

    .section-content {
      flex: 4;
      opacity: 1;
      transform: scaleX(1);
      color: #E74C3C;
    }
  }

  .section-title {
    flex: 1;
    display: flex;
    align-items: center;
    width: 100%;
    margin: 0;
    padding: 0;
    text-align: center;
    color: white;

    h2 {
      margin: 0;
      transform: rotate(-90deg);
      white-space: nowrap;
    }
  }

  .section-content {
    flex: 1;
    display: flex;
    align-items: center;
    margin: 0;
    padding: 0;
    opacity: 0;
    transition: all .25s .1s ease-out;
  }
}

Here is my try and, I don't know why, JS doesn't wrok on this jsfiddle https://jsfiddle.net/CRooY3/krmjqa9r/1/