How do you sue for libel?

libel is like slander, the difference between the two is slander is verbal and libel is written. Hire an attorney and file the claim. You will have to show damages as a result of the libel.

Yes. Corporations (and/or their agents) can be sued for slanderous or libelous behavior. Slander and libel laws vary from state to state, so check with a licensed attorney where you live.

Yes, they have the same rights as any othe citizen.

My sister has written a letter to my other sister with a lot of lies about me. She has been telling these lies for over twenty years, these lies have caused me untold anguish, has alienated my two...

You definitely can, but wouldn't it be easier to just ask them to take down the information you found libelous? They are very accommodating and usually remove the information within 2 working days....