You are not the bioparent and will have to remind yourself of this on a regular basis. Your step child should show you respect as an adult in their life, but their connection with you will be different than their connection with their bioparents. This means you do not get to be the disciplinarian in their lives, it is not appropriate nor will they respect your words the way they will respect the rules and consequences set out by their bioparents. It is more than ok to be a part of the discipline discussion so your stepchild can see that you and their bioparent are on the same page when it comes to discipline. This just means you will be more of a silent partner, standing strong, and supporting your spouse while they lay down the law. Both the bioparent and the step parent are the adults in the household and therefore get to make the rules, however, the bioparent is the only one who gets to enforce the rules.
You will be expected to provide for your stepchild, be that shelter, clothing, food, or whatever else they may need. While legally this is not your responsibility, you will notice your bills will go up with an extra mouth to feed and an extra body to clothe living in your household. As a decent human being, you will find this is a responsibility you will eventually take on naturally, hopefully without complaint. It is important to remember that your stepchild, like all children, is entitled to be provided for. It is not their fault that their parent was a single parent when you met them, they should never hear about the stresses of your financial responsibilities. It is also not their fault if the bioparent outside of your household is either unable or unwilling to step up financially and is not something the child should ever hear about.
Always be a safe space for your step child and be positive for them when they need it most. While you are not the bioparent, you are an adult in their lives. This provides the opportunity to be a support in a different way than a parent might be able to. Your step child may be going through some growing pains that could be uncomfortable to discuss with a parent. You also do not know what is going on in your child's life outside of your home. There could be stressors at school, socially, or in another home you are unaware of. Be a safe and supportive space for your step child so regardless of what is going on in their lives, they have an adult they can turn to when things get rough. Step parenting is not for the weak, but follow these simple rules, and your relationship with your step child will be strong well through their adulthood.