Christian and Hindu marriages

what is the practical difference between Christian and Hindu marriages, in india?

See these three of the most important marriage acts in India in summary form:

A) The Hindu Marriage Act: The Hindu Marriage Act applies to Hindus, Sikhs, Jains and Buddhists. If one of the parties in the marriage is a non-Hindu, he/she must convert to Hinduism before the marriage ceremony can be performed under this Act. People can register their marriage, but registration is not a legal requirement and failure to register a marriage does not affect the legality of the marriage.

B) The Christian Marriage Act: Under this Act, marriages can be performed by a minister of religion in a church or by a registrar of marriages.

(1) If a minister will perform the ceremony, one of the partners must notify the minister in writing of his/her intention. The usual notice period allows the posting of bans on three successive Sundays, but under certain circumstances the minister can shorten this period. After the ceremony the minister registers the marriage and issues a marriage certificate. (2) If a registrar will perform the marriage, he/she will first post the marriage notice in the registrar's office for one month. After the ceremony is performed in the presence of two witnesses, the registrar issues a marriage certificate.

C) The Special Marriage Act (1954): This Act covers marriages between people of different religions, but people of the same faith can also be married under this Act.

Certain papers must be filed with the marriage registrar, including affidavits (in a prescribed form) by each party that they are single, and certifications from the local police station for each party that they have been resident of that area for at least 30 days. A notice of the intended marriage is published.
Thirty days after publication of the notice, the marriage is solemnized by the marriage registrar in the presence of the bride and groom and three witnesses.

The registrar registers the marriage and issues a marriage certificate.
In New Delhi the office of the Civil Registrar, Marriage Section, is located at Tis Hazari Courts (near Kashmere Gate).

Can a third party, for instance if the girl's family is against this marriage solemnised under special marriage act since the girl who is a hindu went ahead with marriage without their consent and knowledge nullify this marriage ?

Normally not if it was legal in all other respects.