I feel like this increases the amount of lucky coincidence needed. Not only I have to randomly get the right mutation, but I also need to have many children (surviving to the age when they can help each other) for reasons completely unrelated to having the mutation. Actually, the mutation may be a bit harmful in the second step, because I may give some of my resources to my siblings instead of my children.
Unfortunately, I am not familiar enough with mathematical models of evolution to evaluate how much this extra burden weighs against your hypothesis.
It seems to me that it doesn't weigh against it very much. A genetic change that causes a not-too-big increase in altruistic behaviour towards likely kin is unlikely to hurt your chances of survival and reproduction a lot.
The first organism with the genetic change doesn't need to be exceptionally well supplied with offspring or anything. (Unless this is an r-selected species for which surviving at all is exceptionally lucky; in that case, it needs to be about as lucky as the bearer of any other not-too-dramatic genetic change has to be.)
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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