The victim is a teenage boy who was under the care of 30-year-old Crystal Ritter, but police say he is not her biological child.
The investigation began in November after someone at the boy's school saw something they thought was inappropriate. Cary police say if the tipster had not called social services, the alleged affair would still be going on.
"We don't deal with this very often. Matter of fact, I can't think of one that we've dealt with quite like this before. This is a different one for us. It's a felony. In our minds it's a very serious one. People are put in a position of trust in cases like this, and we take it very seriously," Don Hamilton with the Cary Police Department said.
Police will not give the teenager's age, but if he were 18 he would be considered an adult. If he were under 16, Ritter would also be charged with statutory rape.
Cary police say if Ritter was not serving in a parental role, there would be no crime they could charge her with.
Ritter is out of jail on $10,000 bond. Without a criminal record, she could face a maximum of 16 months in prison.