Pretend that you have a jar of colored marbles, and that you cover your eyes, reach in, and pull out a handful; then your sibling does the same. Do you think that you and your sibling will pull out the same combinations? The answer is most probably not. It is the same thing with your ancestral DNA.
Yes, you will each get 50% of your DNA from your mother, and 50% of your DNA from your father, but that DNA isn’t passed down in a single block; not every child gets the same 50% from their mother or the same 50% from their father, just like in the little jar of marbles experiment above. Each child will get a random mixture of that 50% DNA (a particular colored marble) from each parent.
If your parents each have 100% of a particular ancestry, (this is not the usual case nowadays), but each has a different ancestry than the other, you and your sibling should each get 50% of each. It’s when your parents have a mixture of ancestry, (usually the case) that you and your siblings will have different ancestral DNA results from one another. Since most people are made up of many ethnicities, the mixture in the marble jar will be more varied, thus each handful pulled out has a higher chance of being different from the previous handful.