You really need to understand the laws in your state. Some would argue that a crime is taking place if there is any romantic component in the relationship at all, whether or not you're having sex. Some consider "dating" a minor to be a crime called soliciting a minor. In some places, being under 16 requires stiffer penalties and enforcement. You need to know exactly what the laws are in your area.
If at some point you tick off the mother or the daughter, someone could have a change of heart/mind about how ok they are with things and pick up the phone. Then you're out of luck.
There's already tension with her mom. Her mind is not at ease. She's afraid of what the court/social services has to say about her role in this. What do you do the day she wakes up and decides the "right" thing to do is call social services or the police?
Do you want to be in that kind of a situation?
If you were my son, I would do my best to convince you to stop seeing her because the potential for danger for you is just too great. There are tons of 18-year-olds out there. There's no need to take this risk.