When a chid has a medical problem that needs to be addressed, that should absolutely be a priority in the family's budget. It is NOT playing favorites. If it was one of your step kids that had the problem, it would receive the same financial attention from you. Make sure the bio mom, and all you kids know this (assuming they are old enough to understand that this bad luck could have happened to any one of them.)
I agree that to go to court is too expensive - financially and emotionally, and can yield unexpected and unwanted outcomes. Been there, done that. (I'm also a step mom of 2, also with 2 bios). What I do suggest is to have a conversation with her, and inform her of your changing circumstances. Explain that ALL four kids will now have less given the problem of the one. Then try to identify some specific things maybe she could take care of for her sons. For example: It would be great if you could buy them their winter coats this year; otherwise the best we can afford for them is 2nd hand on Ebay. Or.... would you mind taking Jr. for his haircut next week? Or....Jr. really needs a pair of soccer cleats, do you think you could take him shopping for them this weekend? Although it doesn't help pay your grocery bills, it at least takes care of osme of their needs so you won't have to.