Try this,
$('#onelvel').click(function () {
if($('.jstree-clicked').length){
$('.jstree-clicked').click();
if($('.jstree-clicked').closest('.jstree-children').prev('.jstree-anchor').length){
$('.jstree-clicked').removeClass('.jstree-clicked');
$('.jstree-clicked').closest('.jstree-children').prev('.jstree-anchor').addClass('.jstree-clicked');
}
}
});
Updated try this,
$('#onelvel').click(function () {
if($('.jstree-clicked').length){
if($('.jstree-clicked').next('.jstree-children').length){
$('.jstree-clicked').click();
} else {
$('.jstree-clicked').closest('.jstree-children').prev('.jstree-anchor').click().addClass('jstree-clicked');
}
}
});