I am trying to find out if my father is really my father...can I do
this by having my brother & sister,as well as myself tested..( I know
my brother & sister are his biological children)

Hello.

Yes, it is possible to use genetic testing to determine if you and
your siblings have the same father.

sources: 

"Brother to brother tests" from DNAnow.com
http://www.dnanow.com/dnanow/botframe_files/brother.htm

"Male Siblings: If the two siblings are male and you are trying to
determine if they share the same biological father than Y-chromosome
testing may be an alternative approach.  The Y-chromosome is the only
genetic information that will be identical in both of the male
siblings as long as they share the same father. If the male siblings
have different biological fathers they will have two different
Y-chromosomes."
source: Genetic Identity - sibling testing
http://genetic-identity.com/Sibling/sibling.html

"Sibling DNA Testing"
source: paternity-testing-service.com
http://www.paternity-testing-service.com/Full_or_Half-siblingship%20Testing.html

DNA Sibling Test, from Americangenetic.com
http://www.americangenetic.com/siblinginfo1.htm

Note that the various DNA testing businesses that I've mentioned are
only listed for informational purposes.  I'm not endorsing them or
recommending them in any way. You should thoroughly investigate any
company before doing business with them. The Better Business Bureau is
a good place to start.
http://www.bbb.org/


search strategy:
dna sibling testing

I hope this helps. Good luck!