i think this is preety obvious but who knows right, so basically all of sanji's brothers his sister are mean but sanji was different they were saying about sanji feeding the rats i think what sanji's father mean about the "rats" was their army. so why sanji's father hates sanji? i think because he wasnt like his brother and his sister sanji respects people and etc etc, so sanji's father was like "this kid doesnt have the power to rule my kingdom, he doesnt deserve to be a vinsmoke he doesnt have the iron fist." but who knows sanji's father might pass the throne to him after sanji beat his azzz or this is one of sanji's father way to teach sanji because he was been chosen ever since to become the heir of vinsmoke