Aside emotions between Poles and Serbs since time immemorial but, you people needs to be aware that it was ONLY Poland that came in aid to Serbians when Ottomans invaded (at that time Russia dealt with Mongols and came later). Polish King Wladislav Warnenchyk and Zawisha Czarny effectively died in the realm of Serbian Empire fighting Turks. Other Europeans at that time? They cooperated with the Turks.
Jadwiga herself was the result of intermarriages between Polish Piast and Serbian Nemanjic houses. Why do you think that nobles organize marriages? To strengthen economic ties. For the same reason Rome prevented Jadwiga to have children. To secure its own and Germanic domination on Baltic-Balkan line that was ALWAYS traditionally ruled by Poles and Serbs.
Jadwiga herself was the result of intermarriages between Polish Piast and Serbian Nemanjic houses. Why do you think that nobles organize marriages? To strengthen economic ties. For the same reason Rome prevented Jadwiga to have children. To secure its own and Germanic domination on Baltic-Balkan line that was ALWAYS traditionally ruled by Poles and Serbs.