Dear questioner, thank you for your important question. We ask Allah to make you firm and assist you to take the next step towards embracing the sound religion of God, Islam With regard to your question, Sheikh Muhammed Salih Al-Munajjid said,

"Congratulations to your brother! Please give him our best wishes and greetings of peace and tell him that even though we are thousands of miles away, we are praying to Allah that he will stand firm in his faith and gain a proper understanding of his religion. Tell him that he has brothers in faith who share his joy at having entered Islam, even though he does not know their names or where they live. This is the relationship of believers with one another, like a structure parts which support other parts.

Dear questioner, you say that you know what the right decision is. This is great. You have come a long way. All that remains is to do it and follow Islam right away. What is life without religion? What is the point of living and working and striving if it is not for the sake of pleasing Allah? Can there be any joy in life or salvation after death or any hope of attaining the bliss of Paradise without entering Islam? If we do not worship Allah, what are we going to worship?? Would a wise person accept to be a slave of his sexual desire that will soon come to an end or be a slave of some wealth that he will soon leave behind in this transient world? There is no need for hesitation in this matter, because it is the matter of salvation from the Fire of Hell and the wrath of God. Also, it is the only way to attain the happiness in this world and the next life. Allah Who created you and created the heavens and earth is greater, and obeying Him is more important than the closest of relatives and the dearest of friends. Go ahead and enter Islam and do not hesitate. Allah will help you with regard to your parents and will give you the strength to stand firm against their pressure. Who knows, maybe you and your brother will be the cause of saving your entire family. Allah told the Prophet Musa (Moses) about his brother Harun (Aaron, peace be upon them both):

“We will strengthen your arm through your brother…” [Al-Qasas 28:35]

So together they went to call Pharaoh and his people to worship Allah. With regard to your girlfriend, do not think that this forbidden relationship will help you adhere to the truth. You have to call her to Islam and to repent to Allah. If she repents, then marry her in accordance with the Islamic way that is acceptable to Allah. If she does not, then do not feel any regrets about her. The Prophet of Islam said: “Whoever gives up something for the sake of Allah, Allah will compensate him with something better.” We ask Allah to hasten the blessings of Islam for you, to bless you with happiness in this world and success in the Hereafter, and to protect you from all evil. We look forward to hearing happy news. Peace be with you. Please, keep in touch…

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Source: Taken with modifications from islamqa.info