Most people don't know what the prefix "step-" means. They are right with their definitions of "stepfather" and "stepmother" but they are wrong with "stepbrother" and "stepsister".
Stepmother: A woman who shares the same husband with your biological mother.
Stepfather: A man who shares the same wife with your biological father.
Stepfathers and stepmothers come from divorce, separation, polygamy, death with remarriage, and infidelity.
Now let's correct your wrong definitions of stepbrother and stepsister.
To make it clear, I've drawn this simple diagram. Let's make it into a story. Note that to make it easier to narrate, I've made all the children male.
Sade never married Ahmed but she got pregnant for him and they had Kabir. Sade became a single mother. When Peter met her, he didn't mind. He married her and they both had Smith and Lucky. Later Peter (Peter ) took Juliet (Julesville ) as a second wife and she too had Pablo and Uche for him. So Juliet had only 2 children and they were for Peter. Ahmed had only 1 child and it's with Sade. Meanwhile Peter had 4 children from 2 different women.
Most of you would say Lukcy and Smith are Kabir's stepbrothers. You would also say they are Pablo and Uche's stepbrothers. You're wrong. Let me tell you the right definitions.
Juliet is Smith's and Lucky's stepmother and Ahmed is their stepfather.
Sade is Pablo's and Uche's stepmother. They have no stepfather.
Kabir has no stepmother but has Peter as his stepfather.
Smith and Lucky are NOT Kabir's stepbrothers. They are his half-brothers and he is their half-brother too. "Half" because you're meant to share 2 parents with a "full" brother but if you share only 1 parent, that's half. Here these 3 brothers share 1 parent - Sade. Thus they're half-brothers.
Similarly, by sharing a single parent in Peter, Pablo and Uche are Smith's and Lucky's half brothers and vice versa. They're NOT stepbrothers.
So are there stepbrothers on this diagram? Yes. There are.
Unfortunately Smith and Lucky have no stepbrothers. There are only 3 stepbrothers here: Kabir's stepbrothers are Pablo and Uche and vice versa. This is how "step" works. Pablo and Uche are Kabir's stepbrothers because they are his stepfather's children while Kabir is Pablo's and Uche's stepbrother because he's their stepmother's child.
This is English language and it's interesting that African languages (at least I'm sure my language Yoruba) have no words for step- relations.
Thank you for reading