Hi, Carol —

Thanks for the question.

Since the main focus of our ministry is evangelization and apologetics, I'd recommend you talk with your local priest or pastor about the pastoral issues involved in your specific situation.
Nevertheless, let me address your questions without getting into the specifics.

You said:

• Is it a mortal sin not to stop someone from committing a mortal sin?
• What if you know it would not stop them anyway or you tried to stop them from committing it, but only passively, instead of actively?

The Catechism gives us the following criteria for a mortal sin.

Criteria for a mortal sin from the Catechism: CCC 1857 — 1860

1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."

1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother." The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.

1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin.

1860 Unintentional ignorance can diminish or even remove the imputably of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The prompting's of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.

We can't control another person's free will; we are only responsible for our own free will.
This includes admonishing the sinning party, your husband, once in charity, and striving not to be an accomplice in his sin.

If this is done, there is no mortal sin to worry about on your part because there is no complete consent on your part to encourage him. Beyond this, I would recommend you bring this issue up, next time you go to Confession. One of the key purposes of Confession is for Jesus, through the priest, to give you counsel and advice.

Hope this helps,

Mike