IOTL Charles IV did have brothers. Furthermore whoever will be elected as the next Holy Roman Emperor/king of the Romans will play a role in the succession in Bohemia; if there are no direct male heirs of the house of Luxembourg, then the HRE will claim it as a reverted fief of the empire and most likely will make his own son the new king of Bohemia.
The effect for Poland will be a surviving house of Piast and no union with Hungary under the Capetian house of Anjou.