This Saturday the journey planners are showing rail replacement buses from Bolton towards Manchester.
There is no fare for Hindley-Leeds-Saltaire, you would have to get separate tickets for different parts of the journey.
If you both travelled together from Walkden to Saltaire via Manchester Victoria and Hebden Bridge and Leeds, you could get a Northern Duo ticket for a total of £21.60 for the two of you (this is the price for two adults: there are no child or railcard reductions). However, you would have to travel together for the whole journey. If it is important that you start from Bolton, then I think that you would need to buy a £21.60 Northern Duo ticket for Salford Crescent to Saltaire for the two of you, and you would need to buy a Bolton-Salford Crescent return ticket for yourself, and your brother would need to buy a Walkden-Salford Crescent return ticket for himself.
There are Hindley-Saltaire offpeak day return tickets which you could buy as you suggested for a total of £41.80 for the two of you (undiscounted adult fares). They would get you by bus from Bolton to Manchester Victoria, and your brother by train from Walkden to Manchester Victoria, then both of you by train to Bradford, you would need to walk between Interchange and Forster Square stations, and then get the train to Saltaire. These tickets are not valid via Leeds.