Yes, the probability of success factors in to the morality of the decision. Prudential considerations do inform the morality of actions. If you think there's a low chance for success, and a high risk of harm done to your family, it would not be a mortal sin not to intervene.
Let's say there are 50 well-armed men about to attack an innocent person If you go intervene, the only thing you'll accomplish is getting yourself killed. In that case, it might even be a sin to intervene, since you're putting your life on the line for no reason. But this is an extreme example. Things could get blurry depending on the circuмstances. If you think you might have a 50-50 shot of saving them, while the penalty for failure would be possibly risking your family ... let's say that if you're killed off, they would go after your wife and children as well, or longer term because they would have no one around to protect them after your death ... then, wow, those become some very blurry scenarios. And you wouldn't have the luxury of time to consult a priest about the scenario.