1. What written legal document should my sister and I make so that after my mother’s death we are both bound by the agreement to divide the property equally.
You can draw a partition deed as per the mutually agreed terms between you both, get the deed executed and registered before the jurisdictional registrar's office.
2. Will such a document supersede the will in case of a later disagreement between us in which my sister tries to take all the property as per the will.
There can be a mention about the Will in the partition deed and how and why it was ignored and also to make it clear that the said will may not be invoked anytime in the future after this deed is acted upon.
3. Will such a document supersede the nominations in the banks FD and demat accounts in case my sister refuses to honor it and wants to execute the will.
The above answer suits this too.