I messed up with my niece, now she's pregnant

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Last year my wife’s aunt gave us a girl she has been taking care of to help us with the housework. She does her work very well but things took a nasty twist for me when I got tempted and started an illicit relationship with her. At the time she went home for the holidays she had missed her period twice and she later confirmed that she is indeed pregnant. Apparently, her mother (aunt to my wife) found a testing kit under her pillow and is now demanding to know if she is pregnant and if yes, she wants to know who is responsible. My wife is already aware of this unfolding scenario and the pressure is building on her to take a test in the presence of my wife. I know I messed up but I need all the help I can get. Please advise...{Vincent}