Kevin boateng is a citizen of Ghana. Why is his brother German?

Their father, a Ghanaian, arrived in Germany very early and gave birth to them in Germany with their German mother. According to German law, they were born German citizens. If allowed by Ghanaian law, they can also have Ghanaian nationality before they are minors. Generally, when you 18, you have to choose which nationality you want to keep and give up the other. If there are special circumstances, you can apply for continuing dual citizenship as an adult. So which country to play for depends on their own choice.