To keep bookmarks folders in alphabetical order you have manually drag it up or down.
To sort bookmarks inside a folder try this.
Hope you have the Bookmarks in folder or folders.
Click the Book icon in the Safari Favorites Bar to show the sidebar.
Drag the Bookmarks folder on to the desktop left of the Safari window.
Drag it back to the side bar and drop where you see the thin blue line.
Now there will be two folders with identical names, one sorted and other not sorted.
Right click the unsorted one and select “Move to Trash”.
Right click the folder on the Desktop and select “Move to Trash”.