Sorry about your fish not making it, I am glad his brother is doing well. Hang in there, when I first got Guppy the first thing he did was promptly get sick. being a new fish mom can be tough, especially when you just jump into the deep end with little help. But it is well worth the trouble. Guppy has been with me for several months now and he is doing just fine in his little 2.5 gallon tank, he is actually becoming quite spoiled!
I have learned a lot about Beta keeping from the helpful and kind folks here, have also been yelled at more than ones and been given contradictory info from different members so I can relate to what you are going through.
People in the pets stores just don't know better, their job is to sell and their training on the actual animals is minimal. I hope your relationship with the one you have left will grow, they really are great pets and not that hard to keep up ones you get the hang of it.