You do not need to do another test.

Download your raw data. (Instructions: http://help.ancestry.com/app/answers/detail/a_id/5557/~/how-...)

Upload your data to www.Gedmatch.com
(Info on how to use Gemdatch: http://dnaadoption.com/uploads/DNAadoption/DNAadoption_files...)

Then you can compare the two directly. Here is some information on how the different relationships between people can be described: http://www.isogg.org/wiki/Autosomal_DNA_statistics

On average half-siblings will share 25% of their DNA, while first cousins share 12.5%. Ancestry's levels are "vague", but many other people have had their half-siblings reported as "close-family-1st cousins" on Ancestry.

Elizabeth
Fredericksburg, VA