Two siblings which died after Mila:[1]

Maternal siblings: A woman who has had two sons who have died after circumcision, is not to circumcise a third son, until he is much older and stronger. This applies even if the sons are not all from the same fathers.[2]

Paternal siblings: Likewise, a man who has had two sons who have died after circumcision, is not to circumcise a third son, until he is much older and stronger. This applies even if the sons are not all from the same mothers. However, some Poskim[3] rule that this worry only applies to maternal siblings and not to paternal siblings. Practically, one is to be stringent.[4]

Third child lived:[5] If the third child was already circumcised and lived, then it is possible that the danger no longer applies.

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[1] Michaber Y.D. 263:2-3; Sefer Shemiras Haguf Vihanefesh [Lerner] 153:1-2

[2] Michaber Y.D. 263:2

[3] Agudah, brought in Rama Y.D. 263:3

[4] Rama ibid; Shvus Yaakov 1:79

[5] Maharit; Reb Akiva Eiger 263; Gilyon Maharsha 263; Sefer Shemiras Haguf Vihanefesh [Lerner] 153:5