No, it will not be Sadqa unless it is clearly specified in the “will” of the person that certain portion of his inheritance will be given after his death as sadqa to the following needy individuals (mentioned in the will) or to the following charitable institutions (mentioned in the will). One can also make a will in favour of any of his relatives/heirs on the basis of any genuine need. Whatever is left afterward, it will be distrubuted to the heirs of the individual after his death in accordance with Quranic distribution of inheritance (More details on the distribution: Discussion 87150 • Reply 87204).
As far as Sawab is concerned: if the individual has left inheritance for his heirs in accordance with Quranic guidelines and also with the intention that this will keep his family afloat and prevent them from deviating from the right path, then Insha’Allah both these factors will earn him reward. There is one Hadith narration in which Prophet (sws) advised one of his companions to not give all his inheritance in charity, rather one should leave enough for his heirs so that they may not beg from the people after the legator’s death and to spend in this way on one’s family earns the pleasure of God and reward for the person.
(Source: https://sunnah.com/bukhari:6733 AND https://sunnah.com/muslim:1628a)