Troubleshooting

Only the medical expenses you claim for your father take his income into account. Expenses you claim for yourself will be reduced by the lesser of $2,759 or 3% of *your* net income. Your brother's claims would be based on *his* income.  

You won't be able to claim expenses that your brother paid, so yes, in the future for tax purposes, you might want to only have one of you pay for your father's expenses. The other person can always reimburse the one who did pay.