Adam Clarke on Leviticus 15:29

Two turtles, or two young pigeons - In all these cases moral pollution was ever considered as being less or more present, as even such infirmities sprang from the original defection of man. On these accounts sacrifices must be offered; and in the case of the woman, one of the birds above mentioned must be sacrificed as a sin-offering, the other as a burnt-offering, Lev 15:30.