Given what it refers to, even if used as a metaphor it is not generally a word I would use in polite company.
brother, father, mother,
but sister, why not sisther?
Father comes from the old Germanic word "fader", mother from "moder", and brother from "broper" (although the "p" is a different pronunciation to the modern "p").
These developed, in English, to the modern spelling and pronunciation during 1500s or so when the "d" or "p" sound was starting to be replaced by "th".
Sister, on the other hand, derives from the old Germanic word "swester", which gradually changed to "sister.
So while your question is valid in that you're looking at the words as being linked through meaning (of family members) and hence might expect a common approach to spelling, their etymology is the key to understanding such things.
I want him report/reports to me as soon as possible.
which one?
Neither.
You want him
to report to you as soon as possible.
With the verb "want", one either wants an object or wants something
to happen.
So while you may want a banana, want an icecream, want top marks on your exam, when it comes to wanting actions to happen you might want to study, want someone to report to you, want her to go out with you, want the dog to not foul the pavement etc.