No, it is not legal for a mother and her son to have a sexual relationship in India.
Moreover, you can be charged with an attempt to rape under Section 376 and Section 511 of the Indian Penal Code, 1860, if your mother objects to the same. You may be jailed immediately. So, you should understand the law and refrain from committing any such acts.
As per Section 376, whoever commits rape, shall be punished with rigorous imprisonment of either description for a term which shall not be less than ten years, but which may extend to imprisonment for life, and shall also be liable to fine.
As per Section 511, whoever attempts to commit an offence punishable by this Code with imprisonment for life or imprisonment and in such attempt does any act towards the commission of the offence shall be punished with imprisonment of any description provided for the offence which may extend to one-half of the imprisonment for life or, one- half of the longest term of imprisonment, or with such fine, or with both.
Also, it is recommended for you to talk about your situation with a counsellor or therapist. They may be able to help you in your given situation.
Reference: The Indian Penal Code, 1860
Section 376, 511 – 202100468-20210012-383