Causation and Affected Siblings

There are three basic ways a condition can be caused:
a) genetics
b) physical environment (diet, injury, etc)
c) psychological environment (parenting, school experiences, friendships, etc)

A lot of people, however, narrow it down automatically to genetics and parenting (nature vs nurture) with 'psychiatric' conditions.
And many of those people also use illogical arguments regarding the presence or absence of similarly affected siblings. It's most commonly used to rule out a cause. But I've heard people say it must be genetic instead of due to parenting because the child's siblings are unaffected, and I've heard the exact opposite.
The truth is that whether or not siblings are affected doesn't really tell you about family-related causes, of which both genetics (excluding random mutations) and parenting are included (and people so often forget that not all conditions are caused by either of those two). Both of those tend to make siblings more similar, but can also account for differences between siblings. The only way to distinguish parenting effects from genetics by studied affected status in siblings is to use twin or adoption studies, and even those don't rule out parenting effects (with twins, identical twins are often treated more similarly than fraternal twins, and adoptive parents may raise a child differently based on what they know of the child's biological parents). The presence or absence of affected siblings doesn't tell you whether it's more likely to be due to genetics or parenting. All it does tell you (and this is not absolute) is how likely the condition is to be due to familial as opposed to nonfamilial effects.
Here are some examples:
A boy is severely depressed, extremely shy, flinches at sudden movements, and doesn't play with other children. His siblings (all brothers) are all apparently normal. The boy I'm thinking of is David Peltzer, author of A Child Called It, whose condition was due to parenting - despite having unaffected brothers. His condition, PTSD, was due to having a sadistic mother who picked one child as a scapegoat and doted on the rest. His brothers actually weren't completely unaffected, but their problems were milder and less obvious, since they were witnesses to violence rather than victims (except for one boy, who became the new scapegoat when David was put into foster care).
A boy is developmentally delayed, hyperactive, and bites his hands when nervous. He talks fast, with odd stressing, and gives strange tangential replies to questions. He has an unaffected brother and sister. This boy is a made-up kid, although I'm sure someone meeting his description exists. His condition, Fragile X Syndrome, is an X-linked genetic condition that his mother carries. His brother didn't inherit the same gene. His sister may have, but if she did, she's an unaffected carrier like her mother - or at most, a mildly affected Fragile X girl who may not be recognized as different, especially compared to her obviously disabled brother.
And then there are the myriad examples of similarly affected siblings, either all raised the same way or all inheriting the same genetic trait. And even more examples of families where two or more siblings are affected but at least one isn't. With genetics, this can be due to not inheriting the same gene or differential effect of the gene due to other genetic traits such as gender. With parenting, this can be due to parents' different reactions to a girl rather than a boy, a first-born child instead of a second-born child, a child with one temperament instead of another, or many other factors. For example, a common pattern in families with a sexually abusive father is for unaffected boys and girls with PTSD, because most male sexual abusers prefer to abuse girls.
If you are theorizing about the causes of a condition, stop assuming that affected or unaffected siblings tell you if the cause is genetics or parenting. Both causes of conditions are compatible with both affected and unaffected siblings.