20 year old is trying to keep my daughter in a relationship with her
Criminal Defense Attorney
I do not practice law in your state and therefore cannot give you legal advice.
I can tell you, however, that in most areas of the United States there are laws against adults (usually defined as being over the age of 18) sending nude pictures to minors (which would be people under the age of 18).
If you have concerns, you might wish to contact an attorney in your area, or someone in law enforcement (again, in your area).
This answer is based upon the minimal information contained within your question. There may be other information which could modify or negate the answer I have given. For particularized legal advice, you're always best advised to consult with an attorney personally, so that the relevant "back and forth" can allow for obtaining all information necessary to a complete answer.