There will be a need to provide enough socio-economic ties for your family members to return home and financial means to support 2 adults for the visitation period. Is your brother a full time carer for your dad back in your home country? Are there any official documents to this effect? A simple factor to consider is how much do you/ your family members have in your bank account/s at this time and how this compares to the total cost of the trip. If the ratio isn't significantly in favour of money in the bank then it will be a hard sell.