You really would assume it would work that way. But the answer is no. Not only is it no, but I explained to him that if him and his sisters took the test, they could get very different results. (In this particular case, he does have an identical twin. Their results should be the same. Our twins, his cousins, are NOT identical, and would likely not have the same results)
That's because although we inherit 50% of our dna from each of our parents, we do not necesarily inherit the SAME 50% that our siblings inherited.
Think of it as an estate. If our farm is split 50/50 between two of our children, one child may take a goat, a cow, and the china. Another child may take 5 goats, a chicken, and a bedroom set. Equal value, but different items.
This is a great article from Ancestry, where 4 sisters took a DNA test, to see "who is more Irish". It helps explain this concept a bit better than my farm inheritance analogy, but without being too in depth or overly technical. :-) https://blogs.ancestry.com/cm/whos-more-irish-you-or-your-sibling/
There's another great article here, with visuals (colored beads), explaining how DNA can be inherited differently by siblings who have the same parents: http://genetics.thetech.org/ask-a-geneticist/same-parents-different-ancestry
Oh, one more thing to keep in mind. The estimates on ancestry are just that - estimates. They are based on the averages, and those estimates keep evolving.