Is it legal for in-laws to keep my jewellery and gifts after separation?

No — jewellery, gifts, and other valuables given to you before, during, or after your marriage are legally your streedhan, meaning they remain your absolute property. Your husband and in-laws have no legal ownership over it, regardless of who is currently holding onto it physically.

You can pursue recovery of your streedhan through a civil suit for recovery of property, and depending on the circumstances — if it was taken or withheld dishonestly — you may also have grounds for a criminal complaint for criminal breach of trust or misappropriation.

Having a clear, ideally contemporaneous, list or photographs of the items given at the time of marriage significantly strengthens such a claim, so gather whatever documentation or witness support you can (family members present at the time, wedding photos showing the jewellery, etc.) before consulting a lawyer about the best route to pursue.