You don't become a parent to your stepchild merely by marrying their parent; you have to adopt the child to become their legal parent. Here are the two main ways of doing that:
Get the Other Parent's Permission
The easiest way to adopt a stepchild is to get both parents permission. In most cases, you will find that the parent you are not married to is not ready to give up their parental rights even though this is necessary for you to adopt the child. This is what are the law says because biological parents have a lifelong right to raise their child as they see fit.
Get the Other Parent's Rights Terminated
The second option, which is way difficult than the first, is to prove that the other parent's rights to the child need to be terminated. This is not easy at all, but there are three ways you can go about it:
Prove Child Abandonment
Fast you may have to prove the other parent abandoned their child. For example, you may be able to prove abandonment if you can prove that the other parent has been missing from their child's life for a couple years or so. However, the specifics of what you need to prove will depend on your state's family laws.
Prove the Parent Is Unfit
The second thing you may need to do is to prove that the other apparent is unfit to raise the child. This makes sense because even though a parent has the right to raise their child as they see fit, the child's welfare or well-being comes first. Therefore, if the other parent has been doing something that is harming the child, then they may get their parental rights terminated. For example, a mentally ill parent may harm their child unwittingly. Another example is a parent who gets drunk and beats up their child; such a parent may also get their parental rights terminated.
Prove the Other Parent Is Not the Presumed Father
Lastly, if the other parent is a man, then you may also succeed in your quest for stepparent adoption if you can prove that he isn't actually the presumed to father. To succeed with this plan of attack, you need to understand the exact elements that make a man the presumed the father of a child in your state. After that, you have to prove that the parent does not meet all the elements required for making the parent a presumed father. For example, in some states, a man is not considered the presumed further of a child if their partner delivers the child during separation. In such a case, you may get the court's nod to adopt your stepchild without getting the other parent's permission.
Speak with a family law attorney to learn more.