The 'Revenge Body' host - who has 12-month-old True with ex-boyfriend Tristan Thompson - was left feeling furious when she stayed awake for their entire flight while her older sister slept, leaving Khloe to take care of her three children too, only for Kourtney to slam her for ''complaining'' too much.
Khloe fumed: ''Get out of my f***ing business. I help you with your kids, I do this and that. I don't have to do s**t. I don't even have to f***ing be here.''
And when the 34-year-old star ''vented'' about having to change her travel plans, meaning NBA star Tristan wouldn't see his daughter for three weeks, in order to accommodate a family Halloween photo shoot, Khloe was unhappy when her sister told her to just ''say no''.
In a confessional on 'Keeping Up With the Kardashians', which aired on Sunday (21.04.19), she said: ''Kourtney is annoying. She's not listening to what I'm saying. I'm not complaining about her, I was just venting to her about a situation. But her comments make me want to slap her in the mouth...
''She's not present, she's on her phone at every waking minute of every day. She's just so f***ing selfish. She doesn't even notice that there's a issue, or maybe she doesn't care.''
After speaking to their mother, Kris Jenner, on FaceTime, Khloe agreed to work things out with her 40-year-old sister so they went for a walk on the beach and she told Kourtney she felt it had become ''hard to communicate with one another''.
She added: ''I feel like we both get defensive. But I want to be able to communicate with you from a non-defensive place, where we could accept each other's opinions. Maybe it's not what we say, it's how we say it.''
The Good American founder complained that Kourtney - who has Mason, nine, Penelope, six, and four-year-old Reign with former partner Scott Disick - hadn't been ''paying attention''.
She said: ''I feel like you just do things so mindlessly -- not in a malicious way, but you're always onto the next thing.''
Kourtney agreed and vowed to be ''more present'' and not let their relationship become ''toxic''.
She said: ''I appreciate your helping raise everyone's kids. I think it's a nice thing to have multiple opinions, if it's said in the right way.
''Everyone has their moments. We just all need to try to do our best to not engage in that toxic cycle.''
His new album Underneath It All is out now.