Solution 1 :

https://jsfiddle.net/7gtcmbjz/

You should just change css

.form .register-form {
      display: none;
    }

to

.form .login-form {
  display: none;
}

Problem :

I’m not really sure what par to the CSS is displaying the the “sign in form” first instead of the “create and account form”

Is it the z-index of the .form class?

.form {
  position: relative;
  z-index: 1;
  background: #FFFFFF;
  max-width: 360px;
  margin: 0 auto 100px;
  padding: 45px;
  text-align: center;
  box-shadow: 0 0 20px 0 rgba(0, 0, 0, 0.2), 0 5px 5px 0 rgba(0, 0, 0, 0.24);

Codepen link: codepen link

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