What do you do if your brother got bitten by a copperhead and will not get medical attention?

There are options, but the nearest emergency room is a best bet. Dial 911 if you have the option. If not, you can tell your brother that he can go with you now and have a fair shot at surviving, or you'll take him after the poison takes hold and he cannot fight you. Remind him that he risks becoming a fatality because of the delay in getting medical attention.

In the mean time, inspect the injury and look for the telltale puncture wounds. If they are there, you know what's coming. At least attempt to treat him for snake bite as you continue in your attempts to get him to go in for treatment.