Answer
Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- Islam gives due attention to the family ties, especially among brothers and sisters.
2- Therefore, you are highly encouraged to maintain good ties with your brother who is one of your maharem (unmarriageable relatives).
Answering your question, Answering your question, Dr. Wael Shehab, PhD in Islamic Studies from Al-Azhar University and currently the Imam of the Downtown Toronto Masjid in Canada, states:
As long as your father has acknowledged this man as his biological child, then he is your brother and is considered one of your mahrams. So, you should keep good ties with him.
As for how to deal with him, you need to deal with him normally as a sister and her brother. Do your best to be a good example for him and to show him Islamic values by means of your practice. Be kind and patient with him, and pray for his well-being.
Almighty Allah knows best.
Editor’s note: This fatwa is from Ask the Scholar’s archive and was originally published at an earlier date.