1. Since his name is mentioned in the deed he is deemed to be its half share.
2. The fact of payment of money by you is of no consideration.
3. Since he is the half undivided share holder in the property , he can gift his half share to your sisters or to anyone he chooses.
4. So you better defuse the tension and convince your father not to do this to you.
5. Or you may buy his share also.
Do note that there is no other option.