12. One who does not mourn:[1]

One who does not mourn a relative as instructed by the Sages, is considered cruel. He is rather to be fearful and worry and perform a Cheshbon Nefesh and do Teshuvah. See Chapter 13 Halacha 9 regarding one who did not keep Shiva/Shloshim, or did not keep it in full, if he should do so later on.

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[1] Michaber 394:6; Rambam