[lambdaphage]

Monogamy should decrease sibling-sibling genetic conflict, relative to polygamy, since you can expect to be more closely related to your siblings on average. Even in ideally monogamous species, though, there is still genetic conflict between siblings, and between parents and children. Even though kin selection can drive these relationships toward cooperation, there still exists an opportunity for conflict whenever a behavior would benefit you more than twice what it would cost your sibling. The tension is heightened for parent-offspring relationships, since the expected reproductive potential of the parent is even lower from the offspring's genes' point of view. In the cases where this doesn't hold, such as in _Hymenoptera_ where females share 3/4 of their variation with their sisters through a quirk of genetics, eusociality tends to evolve and you get as conflict-free a family as you could imagine. A hive. That's what the absence of parent-offspring conflict looks like.

Even during fetal development, in which both parties have a strong interest in the survival of the other, there is a range of conditions that would be acceptable (i.e. better than nothing) to both parties. You should still expect replicating gene machines in such a scenario to claw over the surplus: the mother seeking to distribute her resources among all her offspring in a way that maximizes her fitness, and the fetus to maximize its own.