The Prophet ﷺ said, "Whoever dies while he still had fasts to make up, his guardian (heir) should fast on his behalf." This is understood by many scholars, including the position strengthened by Ibn Hajar and others, to apply to fasts a person was able to make up but delayed until death, whereas one who died still excused (e.g. remained ill until death) owes nothing. Where the family does not fast, some scholars permit feeding a poor person for each missed day from the estate. The narration is authentic and reported by A'ishah.
Evidence — the daleel
Sahih al-Bukhari 1952 ṢaḥīḥSahih Muslim 1147 Ṣaḥīḥ
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.