“My sister, Brittanie, is my best friend, but she’s destroying my life,” says McKenzie of her identical twin.

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McKenzie says that Brittanie is a heroin addict. Fearful of how she may behave, McKenzie says she has told her sister she is not welcome at her upcoming wedding to her fiancé, Chad, giving up her lifelong dream of having her sister be her maid of honor.

“I can’t have that stress on my wedding day,” McKenzie says.

WATCH: How An Engaged Couple Says Her Twin Sister Is Tearing Apart Their Lives

The 24-year-old also says that her sister’s manipulative behavior is causing turmoil in her relationship with Chad.

“I’ve told McKenzie that I don’t want Brittanie at our wedding, but McKenzie keeps holding out hope somehow Brittanie is going to get her [expletive] together,” Chad says. "I need McKenzie to make a choice: It’s either Brittanie or me."

Will this bride-to-be have to decide between her fiancé and her sister? And, what does Brittanie say about her behavior? Is she ready to get sober? See what happens on Thursday’s episode of Dr. Phil. Check here to see where you can watch.

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Young Woman Says Twin Sister’s Fiancé Is Driving A Wedge Between Her And Her Sister