Welcome to the Wefugees Community and thank you for sharing your question with us.
Unfortunately, the freedom of movement of asylum seekers is restricted and they have no right to choose their place of residence. See more information on the German "EASY" system, which regulates the geographical distribution of asylum seekers here. Exceptions may apply for hardship cases and, for instance, for family reunifications. I am afraid, however, that the latter cases only have a chance to get the permission to move to another federal state, if it applies to the very close family (spouses and minor children) and not to (adult) sibblings.
I am sorry that I can not give you more positive information. But I will also link my dear colleagues @mbeon-fardeen and @mbeon-Christine here. Do you agree on my explanations and/or is there anything you would like to add?
In any case, please also don't hesitate to get back to us with any further questions.
All the best,