On the level that we see it, a couple of weeks, which is probably what, a couple of decades time (a guess) family relations can drop, but not that much. But, that is in the real world.
In the sim world, friends can stop being friends in three days, family members can become acquantices and then even have no relationship. Why it would be so hard to renew it is strange to me. My sims don't remain friends, but they are friendly toward each other.
Could having a different traits be a factor in how relationships deteriorate? Sims who are family members and you have a close relationship and similar traits have a relationship that last longer than sims who are close but have different traits? Could that be a factor in length of reltionships?