evolutionary cost to yourself: if you die, then you can't have any more children.
If we suppose that people only consider suicide in bad situations, where, say, their expected reproductive capability is maybe 1/8th that of their surviving relatives, then the evolutionary cost is 1/8th of what you assume. In this case, you would only need to save one cousin, or to have 1/4 of a chance of saving a sibling, or 1/2 a chance of saving a niece/nephew. Or maybe not "saving" a niece, but making it practical for your siblings to deliberately have another niece. ... The scenario remains questionable, and the 1/8 is of course made up, but it does seem possible.
If we suppose that people only consider suicide in bad situations, where, say, their expected reproductive capability is maybe 1/8th that of their surviving relatives, then the evolutionary cost is 1/8th of what you assume. In this case, you would only need to save one cousin, or to have 1/4 of a chance of saving a sibling, or 1/2 a chance of saving a niece/nephew. Or maybe not "saving" a niece, but making it practical for your siblings to deliberately have another niece. ... The scenario remains questionable, and the 1/8 is of course made up, but it does seem possible.