Mariah Yeater Accused Ex-Boyfriend Of Fathering Her Baby, Too

Turns out Justin Bieber's alleged baby mama accused someone else of paternity: her ex-boyfriend. In a wow, pants-on-fire report -- blowing the lid off Yeater's allegations that Bieber is the father of her 3-month old son -- TMZ talked with Frances Lippe, the grandmother of John Terranova, whom Yeater used to date.

According to Lippe, Yeater showed up on the family's doorstep last December and informed Terranova she was pregnant with his child. He denied it, eliciting an angry response from Yeater:

The two got into an argument and police were called ... because Yeater allegedly broke a car window out of rage.

Later that day, according to the police report, Yeater returned to the home to work out a payment plan for the window ... and got into another altercation with Terranova ... and allegedly slapped John 3 separate times in the face with her open right hand.

Yeater was arrested and charged for battery. A court date has been set in Vegas.

As for Bieber, he continues to deny the 20-year-old single mom's paternity suit against him, telling TMZ upon hearing her history: "Wow, that's crazy. ... This stuff is not true. I just try not to focus on it. ... It's just sad that is has to happen like this."

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