I bought my home 12 years ago and am still paying on my mortgage. I want to sell one-half interest to my sister and have her name on the title with me. What is the best way to do this?

The exact way to do this will depend on the state that you are in.  State laws will dictate whether this can be done.  In most states it can be done with an "act of donation" or a "quit claim deed".  The best place to get specific information will be from a reputable title company.  You may want to go back to the title company that originally did your closing or the most recent one if you have refinanced in the last 12 years.  Also note that the sale of your property can trigger the "due on sale" clause of you mortgage.  You should discuss this with your attorney or lender also before transfering the property.  It would be unusual for this to happen but we are in unusual times.