The 28-year-old has rarely been seen in public over the last year and made headlines around the world when he flew home from Kim's wedding celebrations in Paris before her actual marriage took place.
Speaking in the new issue of Rolling Stone, his big sister said: "It's not that mysterious, what's happening with Rob. He has gained weight. He feels uncomfortable being on the show, and that's OK.
"Do I think he smokes weed, drinks beer, hangs out and plays video games with his friends all day long? Yes."
Although Rob hasn't been seen on Keeping Up With The Kardashians lately, he has been discussed. An episode in April saw the family talking about how to tackle his issues.
Kim, 34, had suggested a tough love approach, saying: "Now what we have to do is everyone back off. Everyone licks Rob's ass and does whatever he says and at some point you just have to give it up.
"We all make his life so easy. Okay, we'll drive you around with the chauffeur so no one has to look at you. We have a chef on standby, a trainer, we have someone to run out and buy you socks and underwear. This is pathetic. We are not going to cater to him anymore but it has to be all of us."
Rob himself has spoken openly about his weight gain, tweeting last year that he was aware of his size: "So I found out I was trending for being fat. Thank you all. It really made my day... I'm a grown man and will work on myself when I choose."
His relationship with Kim has been a subject of fascination this year as well.
Rob shocked fans in March when he shared an image of fictional killer Amy Dunne to Instagram, saying it reminded him of Kim. That sparked rumours that all wasn't well between the siblings.
But just last week it seemed things had thawed as Rob reached out to pregnant Kim via Twitter to tell her he'd sent her something to help with her morning sickness.
Reveal is now on Google Plus! Join us here for the latest pictures, fashion and celebrity news!