How do you know if he/she is your stepbrother/sister?

Question

He/she has the same mother

Answer 1

If you all have the same mother but different father, then you would be "half brother" and "half sister" not step. Step means your Mom or Dad married someone that isn't your real parent.

Answer 2

You know if they are your 'step' brother or sister because one of your parents is married to one of their parents. There is no blood relation.

If one of your parents had a child with a completely different person then that child is a half brother or sister. Because you both have blood from one of your parents in common.

Answer 3

your step brother or sister is your dad or mom's new partner's child. in which case the child is not related to you at all except through your dad or mom because of marriage.