"Dear Dr. Fong - My family and I are in dire need of your help. Shortly after we moved into our house my sister thought it would be funny to bring an old gravestone into the house. She found it in an old part of the cemetery which wasn't being maintained. It was broken off so she took part of it home as a joke since it had my initials on it.
Needless to say, no one is laughing now. We awake in the night to strange noises and whispers. Also my sister woke up one night and was being held down by something that she couldn't see. She ran into my room yelling and we searched the house and found nothing.
We decided to return the gravestone and say we were sorry but these strange happenings still continue. We realize what we did was wrong and that we have upset something, but we really need to get this out of our house. I know that you have the knowledge to help me and my family."
This is a fairly simple thing to deal with and I'm sure with just a little instruction from me you will have it taken care of in no time.
What you are dealing with here is your ordinary run-of-the-mill poltergeist. The reason this spirit has settled on your house is not because your sister took the tombstone but because you all feel bad and apprehensive about her taking it. These bad feelings attracted the poltergeist.
To get rid of it just burn some sage in each room of your house and try not to feel too bad about what your sister did. As a side note: if this had really been a spirit, angered because you had desicrated it's grave, you probably wouldn't have been around long enough to write me an email.
Thanks for your question. I am always happy to put my considerable knowledge to good use. If anyone has a question they would like me to answer email me here.