No not illegal to text anyone who is an adult.
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Even if she is living with her parents, if she is over 18 she is an adult so there is nothing illegal about your having contact with her. Her parents may not like it, but its not unlawful...
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While it may not be illegal, it can still cause you problems if they accuse you of something. While you might beat the rap, you probably won't beat the ride. (Having to pay to post bond, get an attorney, fight to get the charges dismissed or tried to an acquittal.) Be prepared to have a lot of drama in that relationship. Good Luck.
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