The answer here is that it depends. Conditions to adopt your spouse's child vary based on the situation. Sometimes, you must obtain consent from the child's other biological parent or guardian and the child themself. Other times, this may not be the case, and you may only need consent from one other person.

In determining whether you will adopt your spouse’s child, consider the following consent requirements and whether you can fulfill the obligations related to your situation.

You must obtain consent from the child you are adopting if the child is over the age of 12. If the child is under 12, you do not need the child's consent, but you may need another party's consent. You will most definitely need the consent of your spouse.

That is, the parent of the child you want to adopt—your current spouse—must consent to the adoption. If the child has not reached the age of 18 but still has surviving parents, you will also need their consent. For example, the person, your spouse, had the child with will need to consent to you adopting their biological child.

Some instances may require consent from more than one person. If your spouse's child has a surviving parent and is under 12, then you will need consent from your spouse, the spouse's biological parent, and the child.

In any event, if the other biological parent of the child who is not your spouse (i.e., the child's other parent) has successfully terminated their rights as a parent to that child, then you do not need that parent's consent as they no longer have any legal rights relating to the child.

But what constitutes consent? In instances where the child is under the age of 18, consent forms require the parent’s name, age, marital status, relationship of the consenter to the child, and the name of the child's other parent.

In all instances, regardless of age, consent requires the date, place of execution, name, addresses, and signature of at least two witnesses and each witness’s relationship to the consenter. The signed consent papers must also be notarized. 

In other words, you may be able to adopt your spouse's child, depending on the circumstance. There will always be consent required of at least one other party, whether it be the child, the child's other parents, or your spouse. You may begin the adoption process once you have obtained consent from the required party or parties.