The answer depends on the personal law applicable to the deceased sister and whether your parents were alive at the time of her death. Assuming she was a Hindu, died unmarried and intestate (without a Will), and her parents had predeceased her, her 50% share would not automatically vest in the surviving sister. Under the Hindu Succession Act, her legal heirs must first be identified. If she is survived by one sister and two brothers, all three would ordinarily inherit her share in equal proportion. Therefore, the surviving sister cannot directly get the entire 50% share merely because the brothers have no objection.
If the two brothers genuinely have no objection, they may execute registered Release Deeds (Relinquishment Deeds) or Gift Deeds in favour of the surviving sister in respect of their inherited shares. Before that, the society will generally require a legal heir certificate or succession certificate (where applicable), the death certificate, indemnity documents, and compliance with its bye-laws. Once the release deeds are registered, the cooperative society can mutate the shares and membership in the name of the surviving sister, subject to its documentation requirements.
If, however, the deceased sister belonged to a different personal law (for example, Muslim, Christian, etc.), or if either parent was alive at the time of her death, the succession would be governed by the respective personal law, and the legal heirs may differ. Therefore, the first step is to determine the applicable law of succession and identify the legal heirs before proceeding with the transfer.