1. Your father is suffering from Dementia for which he is not in a proper mental condition to execute a will or register a deed for transferring his properties,
2. Since all the legal heirs of of your father's properties wish to transfer the property equally in favour of you and your brother, you can get a will executed by your father now (when he is in the phase of having proper mental condition) wherein there will be two witnesses one being your sister and other being a doctor,
3. If you get any deed of conveyance registered by him, it can be challenged later on since it may be construed later on that you have managed to get the said deed registered by your father who is not in his proper mental condition,
4. In case of will it will be different since the title of the property will be transferred after his demise and the other legal heir being your sister will stand as witness of the will along with a doctor to certify that your father was in perfect mental condition when he executed the said will.