This joke shows different understanding of word "brotherly" in Russian and Polish culture. For Russians it means split equally, for Poles it means something else, probably that the big brother takes the largest part. There is similar Russian joke about sharing: Father gives Vova a chocolate bar and tells him: "Go to your brother and share it with him brotherly". Vova: What means brotherly? Father: It means you give him the biggest part. Vova goes to his brother, gives him a chocolate bar and says: "Take it and share it with me brotherly".