You have to look at the purpose of the blessing, and the parts of the blessing that are required for it to be complete.
In a Melchizedek Priesthood blessing for health where a person has been annointed with oil, the person is called by name, the authority is stated, the annointing is sealed, blessing as the spirit directs, and close in the name of Jesus Christ.
Now if the Spirit directs you to say nothing then it is still a complete blessing. With PB there is no oil, but the main purpose is that you receive your spiritual lineage. So if all you received was which tribe you belonged to, then that would be sufficient. One of my thoughts is that we are similiar to our families. If that is true, then a member of the tribe of Reuben will receive the blessings of the tribe of Reuben, and can have the same warnings given to them.
So it is not unreasonable that you have a similiar blessing to a sibling, because you will be raised similiarly, and probably from the same tribe and therefore can have similiar spiritual traits.
Now the converse arguement may be that family can be totally different. The sons of Jacob were all different and received different blessings. Nephi and his brothers were different, and they were from the same tribe. These differences are because of our agency and personalities.
If anyone has any thoughts with regards to my comments, both supportive and contrary to my reasons, I would love to hear from them, as I am constantly expanding my understanding on this topic.
Ben