All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, all perfect praise be to Allaah, Who guided you to Islam and we ask Him to make you firm and steadfast on His Religion until the day you meet Him.
Among the means of being grateful to Allaah for His bounty of guiding you to Islam is to seek beneficial knowledge, do good deeds and befriend righteous and pious people.
By becoming a Muslim, you will only find good, Allaah willing. Therefore, you should ignore the attempts of your brother to divert you from being steadfast on the true religion.
Firstly, we advise you to be patient with the harm that you may encounter from your brother or from other people, and you should be kind and dutiful to your parents and be friendly to your brothers and sisters.
Among the best ways of being kind to them is to call them to Islam and be keen to guide them to it as it is confirmed that the Prophet said: “If Allaah guides one single man (to Islam) through you, it would be better for you than possessing the red camels (i.e. the most precious breed of camels).” [Al-Bukhari and Muslim] For more benefit on calling non-Muslims to Islam, please refer to Fataawa 88906, 84345 and 29347.
Indeed, you did well by wishing good for other people and by being keen on that but it is more appropriate to introduce Islam to your brother and call him to it and clarify to him the good traits of Islam, as this may lead him to guidance and so to repent from homosexuality. You should know that his disbelief is a much graver and more serious sin than homosexuality.
In any case, if you expect that advising your brother would be beneficial to him, then you should continue advising him as long as you are not harmed. But if you expect that deserting him will be beneficial for him, and you think that deserting him is better for you, then you may desert him.
As regards your brother saying to you “you are no longer my brother” then if he means the brotherhood of the family lineage, then this is something he cannot deny. On the other hand, if he means brotherhood in religion, then he is correct as there is no brotherhood between a Muslim and a non-Muslim. The non-Muslims are allies of one another, and the Muslims are allies of one another. Allaah Says (what means): {And those who disbelieved are allies of one another.}[Quran 8:73] Allaah also Says (what means): {The believing men and believing women are allies of one another.}[Quran 9:71]
Allaah Knows best.