Unfortunately, we with family in Nigeria have to pay for the sis of those before us. Unless your father (who might get a Visitor Visa depending on age) may get visa it will be very hard to get one for your brother without a long travel history outside of Africa that required a Visa. You need to show strong ties . The CO has to feel confident that the beneficiary will return wen they are suppose to.
All you can do is apply and let the chips fall where they may. Appointments are pretty far out. But after booking a date you can always keep checking and maybe get lucky and find an earlier date. I did the same for my brother in law.