I strongly recommend the documentary. But "a little off" is an understatement. By the time of the documentary, his highly intelligent brother Charles is still living with his amphetamine-addicted mother in his late 40ies and rarely leaves his childhood bedroom. He admits on camera that he has "homosexual pedophiliac tendencies" and fantasized about killing Robert as a teenager, and complains that he cannot have an erection anymore because of his medication. Tragically, Charles killed himself shortly after filming of the documentary wrapped. Crumbs other brother Maxon [0], an accomplished and highly talented painter who earns money as a beggar on the streets, tries to find some peace of mind by living a zen-like life in celibacy, in a decrepit hotel room, full of meditation on a bed of nails, and is struggling with his intense sexual desire, several sexual harassment suits, and the fact that he cannot physically have sex, because the pleasure is so intense for him that it triggers epileptic seizures. All three brothers appear to have problems with extreme sexual desire, and Robert is the only one who seems to be able to function at all, although the documentary shows him several times in a trance-like, babbling state induced by the sight of a woman (in older age, he once said in an interview that he is "no longer a slave to a raging libido" [1]).
[0] https://www.sfgate.com/entertainment/article/Still-in-the-sh...
[1] https://www.theguardian.com/books/2019/mar/07/robert-crumb-i...