Did my Portal 2 saves get deleted?

So I was playing and was at about chapter 7, I think. All the time I had been relying on autosaves; when I loaded the game after exiting, I just selected "Continue Game" and I just kept on playing.

However, now my brother decided to start a new game of his own. When I went to "Load Save", all I saw were autosaves from chapters 1 and 2. I assume that since I never explicitly saved the game, my autosaves were deleted. Am I screwed?

It does auto-delete autosaves, but it also appears to keep track of which chapters you've completed on this Steam/PSN/XBLA account and when you start a New game it allows you to choose any chapters you've already completed (plus the one you were on).

With most Valve games they've always had a system where they keep a few of your latest autosaves and that's it. When you hit the cap (I'm not sure what it is with Portal, I believe with the HL2 games it was 2 or 3), they start deleting the oldest autosave when a new autosave is created. So yeah, it sounds like you'll have to start over, or at least start from where your brother left off.

Next time it'd be advisable to remember to save your game when you quit.

Yes, your saves were deleted.

One option you have is to go directly to the last map you played -- see this answer for more information.