Op, you have been given a rough time here.
Stealing/taking food secretly despite being told not to, is something that I would want to address too. It does impact.
Kids are part of a household, but should not be given free access to all the food/snacks within it like a lot of posters suggest . They don't have the maturity to fully understand the impact on themselves (teeth/waistlines etc), the budget, the others kids, their nutritional needs etc. There needs to be some level of control. So, yes, just from that view, I would want to address it.
If she is doing this repeatedly, despite being told not to, but otherwise is generally well behaved, then that is worth exploring more and just talking maybe a great place to start .
Anyone suggesting you are a terrible, controlling, food obsessed, disciplinarian and evil stepmother as a result of your posts is simply hysterical.