First things, first – as a stepparent, you have more responsibilities than rights.
But you probably already knew that.
Regardless of whether or not you are legally married to the biological parent of your stepchildren, you are not their legal guardian unless you have a court order explicitly stating so. Or, alternately, you have legally adopted the children, in which case you would have a court order and potentially an amended birth certificate. Also, you probably wouldn’t refer to them as stepchildren.
This can be an impediment if you want or need to engage with government authorities, schools, banks, medical treatment providers and the like. With the exception of serious emergencies, in which case authorities may have permission to act for the care and protection of a child i.e. you take them to hospital with a serious injury.
You otherwise do not have an express right to act on behalf of the child and make major decisions about or for them.
Day to day responsibilities when they are in your care, however, exist.
You can be held legally responsible for the care and supervision of any child in your care under multiple Acts, including child protection and the criminal code here in Queensland.
If a child was harmed in the care of yourself and the legal parent, the child protection legislation can consider you a ‘parent’ under their broader definitions (potentially, even, under the more stringent ones) and deem you responsible for that harm. This could translate to contact restrictions not just between you and your stepchildren, but your biological children also.
Similarly, if you have lawful charge of a child, so the parent leaves your stepchildren with you while they work, then you can be liable under the Queensland criminal code for those children, and are legally bound to provide a reasonable level of supervision to them up to the age of 12.
I think the easiest comparison to make is between you and an educator. When you send children to school or day care, those educators are responsible for the care and supervision of your children during that period. If any harm comes to them, they are the adult legally responsible for your child and can face the consequences if something does happen. And just like you, they aren’t able to make major or significant decisions for the children. Lots of responsibility, limited authority.
Away from the legal position of your rights and responsibilities are your moral and ethical ones.
At the end of the day, regardless of who you are, having a child in your care should be considered a privilege. You have the good fortunate of being entrusted with a tiny human who needs your love, care and guidance to be the best person they can be.
In this context, communicating with the legal parent (or parents, if you are able) is critical, and will enable you to ‘get on the same page’ and work to look after your stepchildren and act in their best interests. It also means that, although you don’t have a legal right to make big decisions for them, you could be in a position in which your input is not only valued, but sought after.
Feed them. Clothe them. Teach them. Guide them.
Overall, be kind and keep them safe. That’s your responsibility not just as a stepparent, but as a human being.