someone already said this

...they're not step brothers, but they're half brothers! i have 2 half sisters who were born after my mom divorced my dad and my dad went to go get married. being step siblings mean that they were both already born by the time the father remarried.

It's obvious you don't fully grasp the concept of step and half siblings. As a person that has both a half sister and a step-sister, I can explain that situation in full detail.

A half sibling is when two children share a "common" parent. Like, they both have different moms, but have the same dad. It doesn't necessarily depend on whether one of the children was born after the father was remarried. Sometimes, you might even have half siblings even if you dad isn't married to that particular person, (*cough* adultery *cough*). Hence the term adults use "illegitimate child" which isn't seen in a political standard anymore.

Then we have step siblings. Now that type of sibling is only acquired by remarriage. It's when two people already have children with their past lovers and marry each other, then the children become step siblings.

If you want a real world example of both kinds of siblings existing in the same family, take my own personal experience. My mother was remarried to a guy who already has a child as well. That child later becomes my step-sister. Then my mother and step-father do the do, and now I have a half sibling. Why? Well it wasn't because she was born after they remarried but because I am directly related to my mother, but have no relations to my step-father at all, hence why I'm only half siblings with that little girl.

I know this was a wordy explanation, but I felt that it was necessary to go into details with this situation.

Since the situation of the characters' parents weren't fully explained yet, we can't have a definite conclusion whether they are step siblings or half siblings.