Sintes,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
div{
display: none;
}
input:checked + div{
display: block;
}
</style>
</head>
<body>
<input name="radio" type="radio" value="1">1
<input name="radio" type="radio" value="2">2
<div>
<input name="radio_sub2" type="radio" value="21">21
<input name="radio_sub2" type="radio" value="22">22
<input name="radio_sub2" type="radio" value="23">23
</div>
<input name="radio" type="radio" value="3">3
<div>
<input name="radio_sub3" type="radio" value="31">31
<input name="radio_sub3" type="radio" value="32">32
<input name="radio_sub3" type="radio" value="33">33
</div>
</body>
</html>