If you read stepmom books or blended family books, you are asked to let things be and respect the other parents' place.
I believe that there is a difference in respecting the other parents' love and place in the child's heart and knowing that one parent blatantly misuses and harms the child. It seems that quite a lot of people out there, that usually fall into the group of mediators and therapists, recommend keeping the children happy, not fighting, and not being upset about anything. I read so many posts about stepmoms feeling wrong or confused because they know that something horrible is happening yet they are told to feel guilty for not just letting the child be.
It is not wrong to teach children values, morals, introduce them to beliefs, introduce them to new things and new ways of thought, and generally be a parent to them. If you are their parent, you will always be their parent. If you parent in a way that is not illegal or abusive, you should be allowed to parent. That is what this state's law says (California).
Instead, court boils it down to which house the child enjoys more (which does not actually mean which house is safest or filled with the most love, parenting and care) and has little to do with parenting. Especially if the child is manipulated or confused and just wants the stuff or to please one parent, usually the parent who makes their love conditional.
If you are seeing that you stepchild is not taking medicine, should see a doctor, needs guidance, can use some cultural awakening, could be introduced to church or new things, can learn about hygiene.... Why should you be afraid? You are in their life for a reason, and they could benefit. The greatest benefit a child could have from a divorce is the introduction of new things from the new people in their lives. Sometimes they won't like the new things, but as any child development expert knows, exposure is key. Without exposure, we do not learn.
Of course, it's best to have your spouse cover as much as possible. But my husband often lets me do the talking when he knows I am more of the expert on a topic, or if something is a little more of a girl issue as we have two girls. I talk to her about bras, we both talk to them about make-up and keeping their bodies covered, we both talk to them about allergies, I primarily talk about food and fitness health, on and on.
Then there are values. Who decides what to teach about hygiene and religion and morals? It only falls on social and child services when it's really too late. Who decides? Parents. If you are a parent, you stay a parent. Your say, your values, your opinions- They go to your child. It is your job. We all know and accept that, except for seemingly ex-spouses and their families. Oh, and family experts who just want the kids to exist and "have fun". The value of simple learning has fallen off the priority list in a lot of articles and books I read...I can tell you I didn't go to two of the best universities in the world by not learning any dedication and values from my parents. I am who I am because of the seriousness of my parents, their support of my hard work, and their encouragement to achieve more. I am very much defined by much of their instruction of values, and I am quite proud of this fact. Not to mention, I know *quite* a few adults and teens who wish their parents had been more value and instruction-driven in their lives rather that concerned about petty or selfish interests.
Your spouse needs to back you, or you need to discuss it first, but this is one of the few places in which sitting back and not taking action is one of the hardest things to possibly do. Why should you not ask your spouse to talk to the child, write an email to the other parent, or contact a doctor if you know that the children are taking the wrong medicine or being ignored in a certain area? The court may not care, but when you married your spouse, you promised to care about him. And with him, come his children. Sometimes you need to not care as much, and I understand that. But there will be plenty of times where children will need guidance and your type of love, whether they respect it or not.
You are a blessing to their life, though they do not know it most of the time or at all. You have a lot to offer, and that is why you are in their life. I'm clearly not advocating suffocation of your lifestyle, but I am tired of hearing and reading silencing messages from non-experts and experts alike. You are not a helpless bystander, but you are in fact tasked with sometimes, or all of the time, caring for these kids. You are important, and your knowledge does differ. They are lucky to have you.