Again, I totally understand your frustration, but at some point you have to let it go.
Junk food is not the end of the world. Yes, it may cause poor eating habits, but again, all you can do is worry about what is in YOUR CONTROL. My girls ate HORRIBLY in the beginning. Now, if we don't have veggies in the house, the girls insist we go to the store and get some. Just don't offer the junk at your home. Simple fix, you know? Biomom will do what she wants to do, no matter what, and getting upset about it and frustrated isn't going to do you any good. Unless she's physically harming your (step)son - and I don't consider junk food to be physical harm
- then you really just need to let it go, for everyone's sake.
I totally understand where you're coming from. I don't know how long you've been a stepmom, but while these issues FEEL like a big deal, they're not. She will feed your son differently. She will discipline your son differently. She will dress him, bathe him, medicate him, and overall care for him differently, and all you can do is accept that as the nature of the beast.