Does your wife understand the situation? I would never allow a sibling of mine or even my brother in law to get away with this. If she is not a fish person, why not sit her down, tell her that just she would not want to see a poor cat or dog die, this is an innocent animal which needs a tank with hundreds of gallons or it will die a slow, painful, gory death. And then have her yell at her brother that he's causing her husband all of this trouble over a fish that he doesn't care about. If he says that he cares about it, somebody needs to pipe in with do you care enough to learn about it and give it room it needs. If you can't make this happen, I really do support the pretending it died thing.