Altruism is defined as

an act that results in

a benefit for the recipient

and a cost for the actor.

An allele associated with altruistic acts toward relatives will spread in a population provided that

C < rB (Hamilton's Rule)

with C = cost for actor and B = benefit for recipient (both expressed as absolute values).

r = coefficient of genealogical relatedness, which is the probability that an allele present in the actor is also present in the recipient as a result of descent from a common ancestor (parent, grandparent).

Benefit and cost are in units of survival X reproduction.   Both are changes in comparison to the absence of altruism.   Benefit is a net gain.   Cost is a net loss.

Hamilton's Rule uses the absolute values of cost and benefit (although a cost is actually a negative benefit).

So it would pay for an individual to sacrifice its life to save the lives of three siblings (or five half-siblings), but not to save the life of only one sibling (or only three half-siblings).

Also it never pays for an individual to behave altruistically toward unrelated individuals (r = 0).

KIN SELECTION SETS LIMITS TO ALTRUISM TOWARD RELATIVES
(ALTRUISM SPREADS ONLY IF C/B < r).

Selfishness is defined as

an act that results in

a cost for the recipient

and a benefit for the actor.

An allele associated with selfish acts toward relatives will spread in a population provided that

B > rC ... or equivalently ... rC < B

with C = cost for recipient and B = benefit for actor.

So it would pay for an individual to kill one sibling if it would save its own life, but it would not pay to kill three siblings to do so.

Also it always pays to behave selfishly toward unrelated individuals (r = 0).

KIN SELECTION SETS LIMITS TO SELFISHNESS TOWARD RELATIVES
(SELFISHNESS SPREADS ONLY IF C/B < 1/r).