There are three kinds of brotherhood:
1- Half brothers/sisters on the mother’s side
2- Half brothers/sisters on the father’s side
3- Full (blood) brothers/sisters
A man/woman is not allowed to get married to his/her sibling if their brotherhood is in one of the mentioned categories. However, if they are stepsiblings not from the categories above, then they are eligible for marriage. For example, if Ahmet’s father gets married to a woman other than Ahmet’s mother and if that woman has a daughter from another man, Ahmet can marry that woman’s daughter.