Like sins of commission (sins resulting from actions performed), the extent to which guilt is afforded by a particular sin of omission is determined by the extent of deliberation involved in the act, and by the magnitude of the precept and the dignity of the virtue that the omission opposes.
[...] The degree of guilt incurred by an omission is measured like that attaching to sins of commission, by the dignity of the virtue and the magnitude of the precept to which the omission is opposed as well as the amount of deliberation.
[2]A person may be guilty of a sin of omission if—as Joseph Delaney states in the Catholic Encyclopedia (1911)—"he fails to do something which he is unable to do" and which he ought to do, because he has put himself into a state or situation whereby he cannot complete the action.
For example, if a person chooses to drink to excess and is therefore unable to perform a necessary task, he is responsible for that failure, even though he is physically unable to perform the task, because he knowingly put himself into a state (drunkenness) where accomplishing the task was impossible.
"[3] In contrast, "Sins of commission are positive acts, by which the negative precepts of God are violated.