Though under Muslim personal law a man can have more than one wife, hardly you see anyone nowadays in India, as such.
It appears you want to have your old flame as a second wife. whether lust alone justifies a second wife as per the tenets of Islam, is a matter you have to discuss with your clerics or get a ruling from established seminary of Islamic scholars.
If you want to present the second marriage as an accomplished fact to your wife, it betrays your apprehensions of resentment on her part for such a marriage. In the circumstances, you have to evaluate the extent of wrath, family pressure, social perception etc, and decide whether you will be in a position to withstand the same.
Also, Prevention of domestic violence against women is universally applicable, and if your wife chooses to file cases against you, you may have to face the situation. You can not hope to get best of both worlds.
You have not stated whether the woman you propose to take as a second wife, was ever married and if so divorced in accordance with law. If not, her conversion to Islam will not have any legal effect on her prior marriage if any.
Since you have not stated any specific grievance against your wife, it is better to have a rethinking on the matter, especially in the context of the age of matrimony being 15 years, which is a fairly long period. Also consider your obligations as a father to the two sons you have, who are grown up and having discerning capacity. Certain psychological aberrations are likely in them on account of resentment against you after your second marriage and apprehensions about their own security. These may take any form including violence.
So, my syggestion is to have a thorough analysis of the matter. Legality alone is not the decisive factor in domestic life.
