Blood is thicker than water, especially blue blood.
Not int he middle ages it ain't. Henry II spent much of his reign fighting his children (and his wife) and this was not atypical.
Your family members were your natural rivals because the fewer of them you had, the more likely you were to inherit something of value.
At Crecy the Black Prince, at all of sixteen years old, held the center of the English line all day long. Edward III watched from a hill. When asked if he wanted to send help to his son and heir, he said "let the boy prove himself a true knight and win his spurs." And they liked each other.
