1. Once you both got married under the Special Marriage Act, 1954 no matter which form of marriage now you both (with second wife who is Muslim) under go with each other, your first marriage under the Special Marriage Act,1954 will ONLY be legally recognized between both of you. This is Law in India and nothing to do with Muslim Shariat Laws that a clergy needs to have replied to your post then it would have been a correct reply !
2. You both (means Hindu wife and you) are now covered under this Act only for the purpose of any matter relating to Matrimonial affairs between both of you.
3. You being Muslim by religion has a legal right to have four wives simultaneously but since she (means Hindu wife) fortunately for you got married to you under this Act she has become bound by the provisions of this Act wherein you now cannot marry second time with any one else till this marriage with her under this Act subsists i.e. till this marriage get dissolved by the decree of divorce.
4. If in case you by mistake or on being misguided by your relatives or any other persons marries again during the existence of this marriage with her (means Muslim wife) then you shall be liable for the offence of bigamy which otherwise you would not have been had she (means here your Hindu wife) married you by converting herself to Muslim Religion then got married with you by a Nikah Ceremonies according to Shariat Muslim Laws. See there is a vast difference of interpretation of correct laws applicability here.
5. As you say her parents are not in favor of your marriage according to the Special Marriage Act, 1954 or even if they are aware of this they don't understand the Legal importance of this marriage which has bound both of you (means you and your first wife) in the surest marriage which the law recognizes irrespective of your Caste, Religion, Creed and Nationality.
6. The very fact she (Hindu first wife) never changed her religion still you both are legally married in the eye of law, her (probable) refusal for conversion to Muslim religion will not give you even a ground for dissolving this marriage on this ground or even if you have a Nikah with second women (Muslim second women is what I mean here) it won't give you a ground to give her (means Hindu first wife) divorce since you did not divorce her (first Hindu wife).
7. What we want to tell you here that the religion of either party to the marriage under this Act has no bearing as far legality is concerned whereas for Religious marriages such as Hindu Marriages which are covered under the Hindu Marriage Act, 1955 or Muslim Marriages which are covered under the Muslim Shariat Law the religion of both parties has to be same, but once you are already married either of these Religious marriages has no legal implications whether you again marry according to Muslim Shariyat Laws or otherwise. These Religious Marriages will just be Social Religious Drama; the Indian laws will only recognize your Marriage under the Special Marriage Act, 1954 for all purposes and in your case you are required to dissolve your first marriage to proceed in which ever form you intend to with next marriage with a Muslim women with a Nikah ceremony and or otherwise as customary appealing to you.
The above is Indian Law in rim and I am sorry I dinot reply as per your liking the very second time. You may ignore my second reply which was made without application of mind!