For fasting, the Prophet ﷺ said, "Whoever dies while owing fasts, his guardian should fast on his behalf," so an heir may make up obligatory fasts (such as an unfulfilled vow or Ramaḍān days) that the deceased was able to perform but did not. For the five daily prayers, the majority of scholars hold that no one can pray them on another's behalf, since prayer is a bodily act bound to the individual; instead the deceased is benefited through duʿāʾ, charity, and the repayment of his debts. If the deceased had a valid excuse for the missed fasts and never regained the ability to make them up, there is no sin and no obligation upon the family.
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.