Q: Does someone who oversaw the father's assets in his lifetime automatically become apotropus of the estate after his death?
A: A person who oversaw the father's assets is not considered as having been appointed by the father after his death, since perhaps the father trusted that the person would not be brazen enough to disobey him during his lifetime. Therefore, only if he appointed him apotropus close to his death do we assume that he intended to appoint him apotropus also afterward (Rama C.M. 290:1; Sma 290:3).
Nonetheless, if it seems to beis din that the father considered the one who oversaw his assets as diligent, beis din can appoint him or her as apotropus over the estate (see Rashba 52a; Ksav Sofer C.M. #22).
If the father appointed someone who was known to be trustworthy, but the person changed his behavior negatively, beis din should remove him and appoint someone else to be apotropus, according to their judgment, since they are "the father of orphans" (C.M. 290:6).
