If you have a gene that makes you help you siblings, your offspring are reasonably likely to get it too, which benefits their siblings (also your offspring).
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.
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