As-Salaamu ’Alaykum sister,
Congratulations on your marriage and may God bless you and your husband, amin.
Your questions have important concerns and I will be answering as a psychologist, who specialized in relationships and family but not as a scholar of Islamic law. (Kindly find below some previous answers of our scholars on similar issues to yours.)
From a strict Islamic legal standpoint, being engaged in any rituals that are outside of Islamic practice is seen as unlawful or highly disliked. At the same time, if you and your husband are not actually saying any prayers or performing any rituals but simply being present in a service, this can be understandable. It is the same as me attending a church for a wedding; it is not unlawful for me to be there, as long as I do not partake in the rituals that could be contrary to my own faith.
As a psychologist, for your family’s sake, you want to look at the big picture. Your husband’s parents are already dealing with the difficulty of “losing their son to Islam”. Perhaps, for his parents partaking in a Hindu marriage is a step of cultural closure and an act of respect for their heritage. With many converts, I have worked with, the family has a tendency to demand the convert to partake in acts or ideas that reflect the family identity. When someone converts, the family can interpret the conversion as “our religion or culture is not good enough, we are not good enough.” Keep this in mind as you continue showing empathy and patience for your in-law experience.
If one day you would like his parents to be open to Islam, then consider this an act of compassion and understanding. They are more likely to appreciate Islam and see it as again in their family. Perhaps one day embrace it themselves. I think in the long run having a Hindu ritual for your in-laws, as long as you do not partake in any exclusive acts of Hinduism, should be a blessing to soften your in law hearts. If it becomes a pattern, where they demand more Hindu rituals and acts, say when you have children, it is important to delineate your boundaries with your husband early on.
As you said, after the marriage you two will have your own lives and be able to build a culture of Islam together as a family. Make sure you two invest in your spiritual growth and talk about future plans of children and how they will be raised as this is central to the religious and cultural differences.
Finally, remember that actions are defined by their intentions. Keeping good favor and kindness never hurt anybody but rather helped people get closer. It is better to error on the side of mercy and compassion than to error on the side of judgment and disconnect.
God knows best.
Disclaimer: The conceptualization and recommendations stated in this response are very general and purely based on the limited information provided in the question. In no event shall AboutIslam, its counselors or employees be held liable for any damages that may arise from your decision in the use of our services.
More from Karim Serageldin
Daughter-in-Law Has Brainwashed My Son