The closer kin, the stranger couple; even the most loving pairs, after spending decades under the same roof, how could they not have any conflicts or friction? The issue isn't the presence of conflict, it's the lack of ability to resolve it.
For instance, elder Brother and Sister-in-law have a great relationship but still manage to give each other the silent treatment over minor household issues.
Why would such a shrewd businessman as elder Brother give someone an opportunity to exploit him? Simply because he was in a cold war with Sister-in-law at the time, his heart was completely wrapped up with his wife, leading to a drop in his guard.
"Ah! I remember now! The reason I was in a cold war with George was because Mina Fernandez had come to me before, saying many things to drive a wedge between us, so—" All of a sudden, Sister-in-law felt the chill of recollection as previously overlooked details came back to her.
It all clicked into place.
