The idiomatic expression "poured oil on troubled water" refers to calming down a difficult or tense situation. However, in the context of the family's fight for the property, it suggests that the patriarchal decision worsened the situation. By "pouring oil on troubled water," the situation may have been made worse by exacerbating old conflicts. Hence, the phrase implies an attempt to rectify the matter, not to worsen it. Therefore, option (4) "to rectify the matter" fits the idiomatic meaning in this context.
Conclusion:
The correct interpretation of the idiomatic expression "poured oil on troubled water" is "to rectify the matter," as it suggests resolving or calming a troubled situation.