I didn't mean to imply that the "overall level" of altruism would remain the same, just that remaining altruist impulses would be distributed less selectively by an individual. If I said a certain society has a more equal distribution of wealth I wouldn't wish to imply they overall also have more wealth per capita. The word indiscriminately was probably not an optimal choice, but I wanted to avoid the warm glow associated with words like "equally", "egalitarian" or "less discriminating".
In an inbreed society you are more closely related (as in likley to have a greater overlap of genes) to your spouse and consequently your children than otherwise. The relative advantage of you being extra altruistic to them, compared to most other less related people, seems greater than it would be in a outbred one. So if I was speculating on how altruism is directed in such a society I'd expect it to drop more sharply when going from me>brothers>cousin>clan member>tribe>ect., because the fitness differences are more pronounced.
Say I have 0.9 Ghandi, and I distribute it 0.5 to my family 0.3 to my tribe and 0.1 to the default human who isn't part of my tribe. If I shift this to 0.4 to my family, 0.3 to my tribe and 0.2 to default human who isn't part of my tribe, this may obviously result in fewer people actually helping each other, because diminishing returns would kick in. What it may also do is for example make coordination on the tribal or even trans-tribal level easier, which can lead to certain tragedy of the commons situations being averted that otherwise wouldn't have.
I want to however strongly emphasise:
I do not have a high confidence in these but they seem to make sense.
We may feel sympathy when we read about people killed for protesting in Syria, Bahrain, Libya, and other countries. But tyranny isn't just something happening to unfortunate people somewhere else. It's an existential risk to human civilization.
Civilization - even tribalism - relies on altruism. Altruism is defined as cooperation that is not the happy convergence of interests of rational self-interested agents. That happens too; but we don't call it altruism. Altruism is, roughly, helping others without the expectation of reciprocation or cooperation. And it happens because humans like helping other humans.
Altruism is probably mostly genetic. It's an evolutionary adaptation that instills the desire to help others into a species. Social pressure can install some amount of altruism; but it's my opinion that this would not work at all without a pre-existing genetic basis. Many species exhibit altruism to a level at least as great as that in humans. Some insects, which are incapable of feeling social pressure, are far more altruistic than humans.
Two theories for how this happens are kin selection and group selection. Regardless of which of these you prefer, both of them have two important weaknesses:
It's not known whether humans are still evolving, or have begun devolving due to lack of selective pressure. But in the case of altruism, we can be sure: Even if some selective pressure still exists, most humans today do not live under the necessary conditions for either kin selection or group selection. Humans are living off their evolutionary capital of altruism.
Tyranny, whether it's that of Syria, Iran, North Korea, Nazi Germany, or the Soviet bloc under Stalin, aggressively selects against altruism. The most-altruistic people were among the first executed in all those places. They are the people being shot while protesting in Syria. Social activism under such a government is rarely in your best self-interest. Tyranny selects for self-interest; people who are willing to help the state oppress others are given opportunities for advancement. And it removes altruistic genes quickly from the population, likely undoing hundreds of years of evolution every year. Those genes will never be replaced.
I'm not too worried when this occurs over a few short months or years. But when a people lives under these conditions for generations, you may end up with a large population deficient in altruistic genes.
There's no solution at that point short of gene therapy. The population can stay in place, resulting in a society that is at best hopelessly mired in corruption and poverty, and at worst a danger to the rest of the world. Or it can disperse, and dilute altruistic genes around the globe.
ADDED: Knowing whether this is a real problem or not, would require learning something about how many genes are involved in altruism, and what their distribution in the population is. A legitimate objection to what I wrote is that if genes for altruism are distributed so that killing less than 1% of the population would have a major impact on their abundance, then they probably weren't very important to begin with. Although, sociopaths are only around 1% of the population, and they have a major impact on society. I wonder how much work has been done in studying the maintenance of alleles for which you only need a few members of the population to have them?