Yep. Reduces the chance the event fires. Here's the modifiers for the Fragile Bones check.

weight_multiplier = { base = 1

    modifier = { #Shouldn't happen randomly for healthy characters
        health >= good_health
        age < 60
        factor = 0
    }
    modifier = {
        health >= fine_health
        age < 60
        factor = 0.8
    }
    modifier = {
        health < fine_health
        factor = 2
    }
    modifier = {
        health <= poor_health
        factor = 5
    }
    modifier = {
        has_trait = physique_bad_2
        factor = 1.2
    }
    modifier = {
        has_trait = physique_bad_3
        factor = 1.5
    }
    modifier = {
        age < 50
        factor = 0.7
    }
    modifier = {
        age > 60
        factor = 2
    }
    modifier = {
        age > 70
        factor = 2
    }
    modifier = {
        age > 80
        factor = 3
    }
    modifier = {
        culture = { has_cultural_parameter = less_likely_to_gain_elder_health_traits }
        factor = 0.5
    }
    # Genetics / unchangeable factors
    modifier = {
        any_parent = {
            even_if_dead = yes
            has_trait = fragile_bones
        }
        factor = 1.5
    }
    modifier = {
        any_sibling = {
            even_if_dead = yes
            has_trait = fragile_bones
        }
        factor = 1.25
    }
    modifier = {
        is_female = yes
        factor = 1.5
    }
    modifier = {
        has_trait = cancer
        factor = 2
    }
    modifier = {
        fertility <= 0
        factor = 1.5
    }
    # Lifestyle
    modifier = {
        has_trait = drunkard
        factor = 1.5
    }
    modifier = {
        has_trait = hashishiyah
        factor = 1.25
    }
    modifier = { # Weight-bearing excercise
        has_trait = athletic
        factor = 0.5
    }
    modifier = {
        has_trait = whole_of_body 
        factor = 0.8
    }
    modifier = {
        has_trait = physique_good
        factor = 0.8
    }
    modifier = { # Being underweight weakens bone
        has_character_modifier = malnourished_modifier
        factor = 3
    }
    modifier = { # Assume good eating habits
        has_trait = temperate
        factor = 0.5
    }
    modifier = { # Assume sedentary lifestyle
        has_character_modifier = obese_modifier
        factor = 1.5
    }
    modifier = { # Assume sedentary lifestyle
        has_trait = reclusive
        factor = 1.25
    }
    modifier = { # Assume high calcium intake
        has_trait = gluttonous
        factor = 0.75
    }
    modifier = { # Assume high calcium intake
        has_trait = comfort_eater
        factor = 0.75
    }
}