They want to get married according to Hindu tradition
Question

                I ask Allah to guide the believers who imitate the disbelievers. My sister’s wedding is approaching, and she has decided to have a party before the wedding that is a pre-wedding celebration in which the bride sits on a chair, with fruit and other food near her, and men and women attend the party to feed her, then they place a special kind of adornment on her head. These are Hindu traditions, in which the Muslims in South Asia imitate them nowadays.
My parents have agreed to that. I hope that you will pray to Allah to guide the Muslims so that they will not persist in committing sins, and so that Allah might admit them to Paradise.
Praise be to Allah.Firstly:
It is clear from the question that there are two things that are contrary to Islamic teachings. The first is imitation of the Hindus, who are disbelievers. It is not permissible for a Muslim to imitate others in matters that are uniquely theirs, such as some types of clothing, celebrations, festivals and so on.
One of the reasons behind the prohibition of imitating the disbelievers is so that this will not have an impact on the thinking of the one who imitates them, because when a person imitates people outwardly, it will have an impact on him inwardly, until he almost thinks of himself as being the same as those whom he is imitating. [Another reason for the prohibition on imitating others] is so that the Muslim will be distinct from the disbeliever, so that the Muslims will not lose dignity and the disbelievers will not be held in high esteem.
Regarding matters such as this, Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said, explaining the situation of those who wear the zunar belt, which was a type of belt that was worn exclusively by non-Muslims in the past:
The Prophet (blessings and peace of Allah be upon him) said: “Whoever imitates a people is one of them.” Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: The least that may be understood from this hadith is that it is prohibited, although the apparent meaning indicates that the one who imitates them is a disbeliever.
Therefore imitating them is not merely disliked (makrooh), because it is an imitation of the zunar belt of the Christians and that would mean that it is haraam, because the Messenger (blessings and peace of Allah be upon him) said: “Whoever imitates a people is one of them.” But that does not mean that he becomes a disbeliever; rather he is one of them in terms of his dress and appearance, which makes it almost impossible to differentiate between a man who is imitating the Christians in his appearance and dress, and a man who is a Christian, so he becomes one of them in terms of his outward appearance.
They said: There is something else, which is that imitating them outwardly may lead to imitating them inwardly, and that is indeed the case, for if a person imitates them outwardly, he will feel that he is in harmony with them, and that he does not dislike them, and that will lead to him imitating them inwardly, so he will  lose his religious commitment.
The correct view is that wearing it is haraam.
Ash-Sharh al-Mumti‘ (2/192-193)
Secondly:
The second reprehensible matter in the celebration mentioned in the question is the matter of men entering upon the bride when she is wearing her adornments, and the free mixing of men and women in the celebration. Both of these matters are haraam.
It was narrated from ‘Uqbah ibn ‘Aamir that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Beware of entering upon women.” A man among the Ansaar said: O Messenger of Allah, what about the in-law? He said: “The in-law is death.”
Narrated by al-Bukhaari (4934) and Muslim (2173).
An-Nawawi said: With regard to the words of the Prophet (blessings and peace be upon him), “The -in-law is death,” what is meant is that the fear of (harm from) him is greater than the fear of (harm from) others, and the possibility of evil and fitnah from him is greater, because he is able to reach the wife and be alone with her, without anyone objecting to him, in contrast to the case of one who is a stranger. What is meant by the in-law here is the relatives of the husband, apart from his father and grandfather, and his sons and grandsons. Those who are mahrams to his wife, such as his father or sons, are permitted to be alone with her, and they are not described as “death”. Rather what is meant here is the husband’s brother, nephew, paternal uncle, cousins and so on, who are not mahrams (to his wife) and   regarding whom people are usually lenient, so a man may be alone with his brother’s wife. This is what is described as death, and it is more appropriate that they not be allowed to approach the woman than one who is a stranger, because of what we have mentioned above. This that I have mentioned is the correct meaning of the hadith.
Sharh Muslim (14/153).
You will find further discussion on the issue of free mixing in the answer to question no. 1200.
We ask Allah, may He be exalted, to guide your family and the Muslims and enable them to give up evils and to hate them, and to guide them to that which is good and right.
And Allah knows best.