What is meant by the impossibility "A Buddha can not help someone not related to him"? What is meant by the impossibility "A Buddha can not help someone not related to him, no relative of him"?

Is said to be one of the three things a/the Buddha can not do. How should that be understood? And what is needed to become one that a Buddha could help? Who is a relative, are the relatives, of the Buddha?

[Related and given in tiven sphere: Buddha can not help one not related to him? Buddha kann keinem "Fremden" nicht helfen?.

(Note that this is not asked for trade, exchange, stacks, entertainment and akusala deeds, but as a share of merits and continue such for release)
Related in this case does not mean by blood. Rather it is in terms of belief in the shared Buddhist philosophies, or at a minimum maintaining an open mind on the subject. Someone not related has no interest in attaining liberation, and such a person cannot be forced or coerced. Thus, they cannot be helped.