Donald Trump had three children with Ivana, and then he cheated on Ivana with Marla Maples, which led to him divorcing Ivana and marrying Marla. So Donald's daughter by way of Marla is a sore spot for the Ivana trio.
And then, Marla quickly divorced Donald. Tiffany was raised by her mother on the far side of the continent, and Donald apparently forgot she existed.
The Ivana trio meanwhile, were sent to boarding school once the first divorce happened, and when they emerged from school as adults they latched back onto Donald
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So the Ivana trio have never been seen as anything other than "real Trumps" (who went off to boarding school for a while), while Tiffany is the bastard child from a homewrecking affair.
The Ivana trio is currently forced to accept Barron, because he (and Melania) is currently Donald's favorite new toy.
I suspect that Donald doesn't hate Tiffany, he just forgets she exists, because they were never close. I suspect that he's dismayed when he finds the Ivana trio plotting against Tiffany, because he wants his children to get along and love each other like a normal family does, but he has no idea how to make that happen, because he's a terrible human being.