AFTER performing together for decades, Donny and Marie Osmond are calling it quits with their hit Las Vegas act because they’re fighting like cats and dogs, stunned insiders dish.

The brother and sister singing team announced they were ending their Sin City residency — which began in 2008 — in November.

Just days before the bombshell breakup, Donny, 61, went on TV and revealed he was furious with his kid sister because she spilled the beans about his secret identity as The Peacock on red-hot FOX competition show The Masked Singer.

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