Placing jQuery UI after Bootstrap doesn't make any sense since they are not dependent on each other at all. But for including bundle before another, you should add dependency to the related bundle.
For custom asset bundle you can just write this:
$depends = [
// Write classes of dependent asset bundles here, for example:
'yii\jui\JuiAsset',
];
But because Bootstrap is built-in asset, you can not modify it that way. Instead you can set it globally through config of Asset Manager:
return [
// ...
'components' => [
'assetManager' => [
'bundles' => [
'yii\bootstrap\BootstrapAsset' => [
'depends' => [
'yii\jui\JuiAsset',
];
],
],
],
],
];
Or just set dependency in one specific place before rendering view:
Yii::$app->assetManager->bundles['yii\bootstrap\BootstrapAsset'] = [
'depends' => [
'yii\jui\JuiAsset',
];
],
Official docs: