Rob Kardashian isn't willing to forgive or forget when it comes to Tristan Thompson cheating on Khloe ... and thinks his big sis can and should do better.
Sources close to Rob tell TMZ ... he's still furious with Tristan, and regardless of the couple seemingly getting back together and working their relationship out -- he's not having it.
We're told Rob thinks Tristan's an "a**hole" ... and he'll never be able to let go of him disrespecting and embarrassing his closest sister while she was pregnant.
Rob's apparently not on board with the rest of the family's approach of not pressuring Khloe to dump him either. We're told he's adamant she deserves better and should move on.
There is a bright side, though -- our sources say Rob's excited to spoil baby True, who he says is "gorgeous," and believes Dream will always look out for her ... like Khloe did for him.