Ruling 2394
A father or paternal grandfather can wed to someone his non-bāligh child/grandchild or his insane child/grandchild who has become bāligh while in the state of insanity. After the child becomes bāligh or the insane individual becomes sane, if the marriage is detrimental for them, he/she can either approve or reject it. But if such a marriage is not detrimental and he/she annuls the marriage after they become bāligh [or after the insane individual becomes sane], the obligatory precaution is that they must either get a divorce or conclude another marriage contract.[1]
—Ayatullah Sistani, Practical Laws of Islam, Marriage
1
The interpretation of this ruling is based on Ruling 980 of al‑Masāʾil al‑Muntakhabah (p. 362).