Hello Loic!
To change the opacity of points you can use a
point.update method. In the Sankey chart, you should call this method on nodes. Please, take a look a the demo below to see the example.
Demo: https://jsfiddle.net/BlackLabel/619hovL8/
API Reference: https://api.highcharts.com/class-refere ... int#update
Feel free to ask any further questions.
Kind regards!