Leghorns are considered one of the least likely to brood over eggs, of all chicken breeds. Leghorns (in general) will live and produce eggs without EVER going broody. In my experience, NEVER GOING TO HAPPEN. However, I must add that even though the instinct to brood has been almost eliminated from the Leghorn breed, it is still part of chicken instinct. There is no way to guarantee that Leghorns will or will not brood. I have found that in my 15 years with chickens, that Leghorns simply don't do this. I have several Red Sex Links that are half White Leghorn and Half Rhode Island Red, in three years with them never have I had a broody hen. It's possible that a hen (leghorn) will try to brood but usually they don't keep the eggs warm enough to hatch (leave the nest too much for too long) or that they make poor mothers.
Isa Browns are in a similar category as Leghorns. They are a sex-link chicken and are bred for high egg production. In the development of both these breeds, broodiness was culled out of the breeding stock to increase egg production. A broody hen won't lay for at least 6 weeks if stopped. Leghorns are considered non-setters while Isa Browns are considered rare setters. It is more possible with this breed but also unlikely. My Sex-Links are Golden Comets but also come from RIR stock. RIR (and whites) are poor setters. You have nothing to worry about.
I would say, based on my knowledge and understanding, that out of a hundred of each breed, you MIGHT have one attempt going broody.
Maybe I have an exception...she's a wee little bantam Leghorn who has been repeatedly broody over the past couple of years I've had her. I've always discouraged her; she's a faithful layer of perfect, pint-sized eggs when not broody. But - just yesterday, a friend gave me seven, fertile, bantam Plymouth Barred Rock eggs for #7 bantam Leghorn to set on. #7 had been brooding an empty nest for a few days before I slipped the eggs under her. She's doing a great job so far - leaves the nest once a day for about twenty minutes to eat, drink, and dust bathe, then she's right back on it.
Here's an old picture of #7 & #8 before #8 was et by a fox:
Matt - I love my bantam Leghorn(s). I live in a cold climate, so have to keep an eye on their combs in the winter. Good ventilation in the coop is definitely essential, and I change the pine shavings out frequently to help decrease the moisture. I used Vaseline on their combs last winter, and left a UV red bulb burning during the coldest weather. Also, I've read that it's more massaging the comb than the actual Vaseline that helps their combs from becoming frostbitten.
#7 is still in full broody mode. I'm interested to see how her mothering skills are...