A DNA Sibling test will show you whether siblings are full siblings (same mother and father), half siblings (only one parent in common), or not related.
A DNA sibling test is not as powerful as a DNA paternity test and should only be considered when it is absolutely impossible to test the alleged father (if the alleged father is missing, deceased or absolutely refuses to be tested). While a DNA paternity test will also give you an absolutely conclusive answer regarding paternity, sibling tests can only give you a likelihood ratio which indicates the likelihood that siblings are full, half or not related.
How to read a DNA sibling report
