I've been using Chrome for years, but I never used the "sign into chrome" feature, I always just signed into individual services. My brother used my computer and signed into his Chrome account on it, and Google assumed he was me signing into chrome for the first time and automatically deleted all my saved passwords, bookmarks, and history from my computer and synced it with his Chrome account. Now I'm left with a wiped clean google chrome and he has all my saved passwords...which is a huge pain in the ass for me.

Is there any way to undo this?

Nope, since you never had them backed up (ironically by not using the same feature he did), that's all there is.

He can wipe all out of his synced data on Google, but there's no way to just roll back what you did.

fraid so. Next time use Chrome Sync, or use the multiple user settings in Chrome, or use separate user accounts, or perform regular backups.