I imagine that Josie is the only one with a gun licence and therefore the one that would be charged in relation to anything gun related at the home or property. In a healthy home and relationship, even on a farm, a gun is not an item you use every day. You would likely use it for pest control and for putting down any animal in need - sickly sheep, injured dogs etc. Therefore it's not likely that everyone on the property would need to use one. When it came time to use it, the person with the licence would do the deed I believe.
It appears Josie was the farm worker, and while Jess was around at least, Shannon was the caregiver of the children. In my mind, when it came to physical labour, that deems Shannon the least useful, or strong, of the 3 women on the farm We do not know when or even why Jess came back to the farm, the reason hasn't been stated as fact. It could have been because she broke up with Josh and needed somewhere to live, but maybe she was summoned back to help - maybe because it was a busy period (possibly shearing season as it would happen around the time of Gus' disappearance). If she was summoned to help it could mean she took over Shannon's role - but why would you want her back to help as it doesn't add another set of hands, it just replaces one as the kids need caring for? Or was Shannon beyond being able to help anyway, so Jess was another pair of hands? Was Shannon declining and unable to work the farm any more, so Jess was needed? Or was it merely because Jess moved away from Josh and either helped Josie, or replaced Shannon?
Necessity is a reason for doing many things on a farm, and because of that it's not always the best person for the job, or even an adequate person for the job, it's pitching in and getting things done any way that works. For that reason, in an ideal world, maybe Shannon would not be deemed a suitable care giver but you just have to get on with it on a farm. The sheep, fences, dams, shedding etc doesn't care if you're aging, have a bad back, are sick, have dementia etc
MOO