Nope. It isn't strange. It is the right of the heir to determine what will be. Custome or normalcy has nothing to do with it. It isn't normal for people to have their noses changed to look completely different than what they were born with. By your logic, one with lack of information to the contrary could look on Michael Jackson for the first time and reason that it must be his nose as it was when he was born because it isn't normal for people to alter them. You rule out all alternatives on the basis of maybes and rule in other alternatives on the same basis when you have no real ground to begin questioning the validity of the Bible. The Bible says Jesus had brothers and sisters. The people pointed out that these brothers and sisters are of the same line ('And are not these "HIS" sisters..') etc.
Fact is, you are relying on a pile of if's and wishes and there is doctrine that the RCC has built upon as much. The doctrine proclaims itself as truth in absence of any supporting evidence and inspite of loud evidence, nay, truth to the contrary.
Sorry, Havoc, but I have to disagree with you on this one. The obligation for children to care for their parents is a matter of God's commandment. The firstborn cannot arbitrarily remove this obligation from his brothers.