Nuclear DNA is found in the nucleus and contains all the genes that make up who we are. Nuclear DNA is the type of DNA that police use to profile a person, and what DNA labs use to perform a paternity test.
Sperm cells have mitochondria in the tail of the sperm, and not in the head of the sperm. When the sperm fertilises a egg only the sperm head enters the egg, and the tail is dropped off. Thus, children will only receive mitochondrial DNA from their mother. Testing the sequence of mitochondrial DNA in siblings will show if they are biologically related via the maternal line.
Thus mitochondrial DNA testing is a very powerful and accurate way to test for biological siblings. Mitochondrial testing is currently used by military DNA laboratories to identify skeletons discovered in old war zones, by tracing the mitochondrial DNA back to living relatives.
The US military is still using mitochondria DNA testing to identify bone fragments from world war II, Vietnam and Korean wars, and many other conflicts.