All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If the marriage contract you and your husband concluded, while you (the wife) were still Christian, before you became Muslim, fulfilled the conditions of a valid marriage; i.e., if you were chaste at that time, then your marriage was valid. As for the pre-conditions that are not regarded in the Sharee’ah, such as giving you the right to divorce him, or the like, then they are of no value.
The conditions of a valid marriage in Islam are the presence of the guardian of the woman or his consent, the presence of two witnesses, the Mahr (bridal money given to the woman).
For more benefit on the conditions of a valid marriage, please refer to Fatwa 83629 and 86755.
Now, if you were not from the People of the Book (Christian or Jew), or you were not chaste before marrying him, or if any of the conditions of a valid marriage was not fulfilled, then your marriage is null and void.
However, if your marriage was not valid according to the above and you want to be the wife of this man, now that you have become Muslim, then you should make a new contract that fulfills the conditions of a valid marriage in Islam.
On the other hand, it is unlawful for this elder brother to compel his younger brother to divorce you, if the marriage you did before you became Muslim was valid. It is also unlawful for him to prevent his brother from marrying you if the first marriage was not valid.
In fact, the reason he gives for his demand is baseless. There is no discrimination in Islam. Allah Says (what means): {O mankind, indeed We have created you from male and female and made you peoples and tribes that you may know one another. Indeed, the most noble of you in the sight of Allah is the most righteous of you. Indeed, Allah is Knowing and Acquainted.} [Quran 49:13]
Abu Hurayrah narrated that the Prophet said: “A woman is married for four things: for her wealth, her family lineage, her beauty and her religion. So choose the religious one, you will prosper.” [Al-Bukhari and Muslim]
The Prophet also said: “Allah, Most High, has removed from you the pride of the Pre-Islamic Era of Ignorance and its boasting in ancestors. One is only a pious believer or a miserable sinner. You are sons of Adam, and Adam was created from clay. Let the people cease to boast about their ancestors. They are merely fuel in Hellfire; or they will certainly be of less account with Allah than the beetle which rolls dung with its nose.” [Ahmad]
So, we advise you to try to convince this man not to accept this pressure from his brother. You can seek the help of some righteous people who could act as intermediaries between you and him and could talk to his brother and family warning them against interfering in marital relationships.
Indeed, Islam forbids interfering in such relations except for the purpose of strengthening them.
But if you cannot convince this man and his family to pursue in this relation, and you come to the conclusion that this man wants divorce and separation, then you have no right to prevent him from doing so. Allah will surely compensate you with a better husband as anything you give up for Allah's Sake, Allah compensates it with something better. Allah Says (what means): {…and it may be that you dislike a thing which is good for you and that you like a thing which is bad for you….} [Quran 2:216]
Allah also Says (what means): {And whosoever fears Allah and keeps his duty to Him, He will make a way for him. And He will provide him from (sources) he never could imagine. And whosoever puts his trust in Allah, then He will suffice him.} [Quran 65:2-3]
Allah knows best.