I met my 15 year old sister recently and many people say she looks like me a lot. I always knew she was my moms kid but never thought she was my dads. They where divorced long ago but not before she could have been concived.
I know there are tests that can be taking but is their one that can be took between myself and her to see if we are half, or full? My dad doesn't know of her so one between her and him is out of the question.

Thanks for the help.

Sure. A Y-Chromosome test will confirm if you have the same father. They run about $150 per test. There are a variety of companies/organizations that offer it on the net:
http://www.google.com/search?num=100&hl=en&lr=&newwindow=1&q=y-chromosome+dna+test&btnG=Search

And this is something her and I can take without involving my dad?

Yes. The test confirms whether you two share fathers or not.

But Rick, do they both have to be over 18 or have parent permission?

J_9,

Even if they require parental permission, they can get their mother to sign off, thereby never involving the dad.

I was not sure that Mom was involved since he just met his sister that is his mother's child.

Good point.

Well, either way, odds are no parental permission is needed (or they can forge it).

Not sure where he is, but if here in the States forging is almost impossible unless - taking another adult who will forge for you.

Thanks for the help