The 32-year-old sock designer filed documents in Los Angeles earlier this week asking to be the main carer of three-year-old Dream - whom he has with Blac Chyna - and sources have now said he has the support of his older sister Khloe.
An insider told People magazine: ''Khloe has always been very close with Dream. When Rob had custody of her as a baby, Khloe often helped out. She really loves Dream.
''Khloe will do everything she can to help Rob get custody. She thinks this is best for Dream.''
Khloe, 35, has a close relationship with her nieces and nephews, including Dream, and recently documented a playdate between Dream and her own daughter True, 20 months, on social media.
The 'Keeping Up with the Kardashians' star has always been close with her brother, who lived with her for several years.
In his documents, Rob asked a judge to limit the time Chyna spends with their daughter to just weekends with a nanny present as he believes she's a danger and isn't behaving appropriately around their little girl.
The reclusive star accused his former lover - who also has seven-year-old son King Cairo with her ex-partner Tyga - of regularly hosting parties at her home in front of Dream and invites strangers round to get drunk with her.
Rob believes Dream's behaviour has changed recently as she's started ''naked twerking, and acting out sexual positions that she said her mom taught her'', as well as swearing and using language he insisted he never uses around her.
He's also said his daughter arrives to him with messy hair and dirty teeth.
Rob's paperwork states that Khloe can back up his allegations as she's noticed a behavioural change in Dream as she's ''more in defence mode'' and is ''decidedly more aggressive'' during her play dates with her cousins.
However, a lawyer acting for his Chyna has blasted Rob for moving to take the tot away from her ''hands-on, extremely loving'' parent.
Lawyer Lynne Ciani said: ''So Rob Kardashian - who has ... posted illegal revenge porn against Chyna - wants to take custody of Chyna's beautiful daughter, Dream, away from her hands-on, extremely loving mother?.
''And Khloe Kardashian ... wants to take her niece Dream away from her loving mother, Chyna? Oh, please.
''Chyna will vigorously contest this latest despicable attempt to take away her daughter in a court of law, where she will keep prevailing against her ex-fiance Rob and the other malicious and vindictive members of his family.''
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