In the Hindu family system, it is a normal proactice that the father will take up the financial responsibility of his daughter's marriage. It has become such a universal and common pracice that no need was ever felt to make such a practice mandatory by law. It is therefore advised that you and your sister must request your father to contribute towards your sister's wedding.
There are two possible legal options you can look into to help you in the current situation -
In case your father has an ancestral property you and your sister as the coparceners of the property can demand for partition of the property and can use your sister's share for the marriage. The other possible option is to file a case against your father under the Protection of Women from Domestic Violence, 2005. Your sister must firt request your father to finance her wedding and if he refuses then she may file a suit for domestic violence. Under 3(a) of the Protection of Women from Domestic Violence, 2005 economic abuse has been identified as a form of domestic violence. The defintiiton of the term economic abuse is quite wide in the act and may be employed to cover your case. For this you will have to convince the Court that your father had the responsibility towards your sister's wedding which he failed to perform thus amounting to economic abuse. In this scenario, it is best that the suit must be filed by your sister, as an unmarried daughter, she has a domestic relationship with your father which is reuqired to file such a suit.