Dear @Ali Naweed,
I am assuming that your parents are not currently in Germany.
Minor asylum seekers (under the age of 18) in Germany without parents or guardians are as a rule taken into the care of the children's office (Jugendamt). They are classified as 'unaccompanied minor refugees' (unbegleitete minderjährige Flüchtlinge). Having a brother or sister over the age of 18 does not change this. These rules are in place first and foremost to assure the welfare of the child.
If, however, your sister can show she has been given responsiblity for your brother by the parents/guardians, she can be given parental custody as a temporary measure. She needs what is called a 'Sorgerechtsvollmacht' (roughly meaning 'power of parental custody') from the parents/guardians. It would allow her to make decisions on his behalf regarding his welfare. It would also allow them to stay together in suitable accomodation. The Jugendamt would have to check this document to see whether it fulfils the requirements.
Here is an exmaple of such a document in different languages (the document doesn't need to be in a specific form, this is just an example):
Even if she cannot show that she has parental custody, the Jugendamt has to take the welfare of the child into account. Seperating them would not benefit the child so they have to **** at what arrangements can be made so that they at least have regular contact.
If she would like to know more about this, I recommend she contact the following organisation. It advocates for the rights of such unaccompanied children and has experience in dealing with such cases.
Here is information from their site about such cases:
And here is information on how to contact them (they speak English):