Slammed by First Son's Family ...

Tristan Thompson Slammed by Sister of First Son's Mom, Says He's a Bad Father

Tristan Thompson Only Shows Up for His Kids with Khloe

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Tristan Thompson may be getting praise from the Kardashian family for being a good dad, but it's certainly not that way across the board ... as he's now getting slammed by the aunt of his first son.

Tristan has a 6-year-old son named Prince with Jordan Craig ... and Jordan's sister, Kai, just unleashed on the former NBA star. Kai says it's disgusting Tristan can't show up and be a "real parent" for Prince yet he can do a bunch of nice things for the two kids he has with Khloe Kardashian.

Kai continues, "The fact that he can take other children to school and…