What is the purpose of these winglets on the Chinese version of the An-2?
As you said, the Y-5 and Y-5B are Chinese variants of the Antonov AN-2. The Wikipedia page gives this info on the Y-5B:


  Shijiazhuang Y-5B-100 – Y-5B aircraft fitted with triple tipsails on the upper wing tips, which reputedly gave 20% higher climb rate and improved L/D ratio by 15%.


Here is a research paper on tipsails which may help a little bit.

There doesn't seem to be a lot of empirical evidence of the performance gains that China claims with the Y-5B. I can't find examples of the triple tipsail design used on any other aircraft.

Just look at the aircraft though, all the struts, wires, hinges, etc hanging out in the wind, and these 3 little tip sails improve L/D by 15% and climb rate by 20%? That's a hard pill to swallow. If it were that simple, it would be a feature of nearly every aircraft.