Purity: Qualifications of the Guardian (Wali)
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 535:
The guardian of a deceased person must not be a non‑bāligh child nor a person who is insane. Similarly, a person who is absent and cannot personally undertake the duties or instruct someone else to do them cannot be a guardian.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)