How long did it go without food? If the tank has a filter is everything still running? I don't think forgetting to feed for a day or two will kill a goldfish.

Tip: don't use the term 'it's just a pet' around a pet owner. If you ever end up with a pet, then you'll understand.

Tell him the truth.........

agree with everything annztoo said. and yeah, "just a pet" doesn't set well.

IF this post is not a fake:

Man up, don't lie.

"I don't want him to think he can’t rely on me for something as simple as feeding his precious goldfish." This is exactly what happened, your brother cannot rely on you for something this simple, and now you're considering lying about it. Seriously, is this how you want to live your life?

Also, your use of the words "his *precious* goldfish" --whether intentionally snide or not -- displays an attitude about how dismissive you are of a pet that your brother cherished.

It might not be your fault, who knows? You must not lie. Be honest, go to the pet store with your brother and replace the fish.