New Delhi, Nov. 25 -- Philippine President Ferdinand Marcos Jr. refused to respond on Monday to an allegation by his estranged sister, a senator, that he has been a longtime drug addict whose cocaine dependence has undermined his governance, saying with a somber tone that he didn't want to discuss a family rift in public.

Communications Undersecretary Claire Castro has said that Sen. Imee Marcos' accusations against her own brother before a huge religious rally in Manila last week were "a web of lies," and may have been a desperate attempt to distract ongoing investigations into a corruption scandal that may implicate her opposition allies in the Senate.

Aides have said in the past that Marcos Jr. had tested negative for cocaine and met...