Why it is generally assumed by people and society that it is always a sister who is jealous of brother's wife but ain't the brother's wife could be a trouble maker in brother-sister relationship?

Brother Sister relationships can be distant, close or even intimate. It is common for a brother and sister to form a type of relationship similar to a courting pair, IE. boyfriend - girlfriend, without any physical or incestuous interaction. Either one could feel some jealousy and even rejection if a close relationship is suddenly altered to accommodate a new person. The new person may not be aware that they are displacing the other sibling, and neither might their potential partner. The only one typically, and sometimes painfully aware of the new stress is the other sibling.

That sibling is likely to be the one that instigates tension with the new "girlfriend" or "boyfriend" but the response may be out of proportion to the initial threat. Therefore the "culprit", as in any conflict, is the one (or more) that escalates the hostility out of proportion to the threat.