Though he has yet to publicly address the news, according to a source, Rob, 31, is “furious” at Thompson, 27, for allegedly cheating on his sister Khloé Kardashian, 33, throughout her pregnancy.
“Khloé has always been there for Rob and he hates that she was hurt — he knows what it feels like to be screwed over by someone you trusted,” says the source of Rob, who has an infamously rocky history with his ex, Blac Chyna.
Adds the source: “Rob won’t forgive Tristan for this.”
Khloé and Thompson welcomed their first child together, a baby girl named True Thompson, on Thursday — just days after allegations surfaced that the Cleveland Cavaliers player had been unfaithful to the reality star multiple times.
And Rob’s not the only member of the famous family outraged by the news: On Tuesday, a source told PEOPLE that Kim Kardashian West, 37, and her husband Kanye West, 40, are also upset with the NBA star.
“They both feel Khloé has gone through so much and that she deserves being happy,” said the source. “They can’t believe that Tristan has been going behind everyone’s back. They find his behavior disgusting.”
“Kim is very protective of Khloé and is especially upset,” added the source. “She can’t imagine Khloé will stay with Tristan, but will support her sister either way.”
RELATED VIDEO: Why Khloé Kardashian’s Daughter True Has Tristan Thompson’s Last Name
Thus far, the future of Khloé and Thompson’s relationship remains unclear, but sources have told PEOPLE the new mom is choosing to focus on her newborn.
“Khloé really wants to be able to forgive and trust Tristan again and have the life she hoped to have with him,” one insider told PEOPLE on Monday. “She doesn’t know if she’ll be able to do that, but that’s the goal. She loves the life they had, she loves the baby they have together, and she still loves the dream of being a family together. If she can find a way to rebuild with Tristan, she will.”
And according to the source, the Keeping Up with the Kardashians star never considered giving her daughter her own last name.
“Khloé still very much loves Tristan, and this baby is one half his,” said the source. “True is their child together. Giving the baby his last name was never in question.”