Please Help me!

So I'm working on a project in a class called IT Systems, which is basically a dorky computer class everyone is required to take in order to graduate right? So were working on this poster thing, where we have to explain the process of how information from Google gets to your computer, right? So it didn't work out for my original partner. She was a fat, (extremely) ugly, troll who didn't do any work at all and wanted me to do it. I was like "Honey, hell to the no. Fail on your own." So I got assigned a new partner, who happened to me my brother's best friend!
So we start talking, he doesn't understand it so I'm doing a lot of the work, I mean he does some, but not much. The difference is I can tell he's trying. Kind of. Okay, he's not but the difference is that he's cute. Which is one of the reasons why I'm not dumping him like I did the troll.

Reasons Why Not To Leave Patrick Alone On Our Project

1. He's cute. Of course.
2. He's my brother's best friend. Only bad things come from making your brother's best friend mad at you.
3. He's pretty cool, he plays football too which is an added bonus.
4. He cares about me.

Yes. He cares about me, he's making it completely obvious with points you can't miss! He stops me in the hallway just to say hey, he opens doors for me when he's around, sappy stuff like that. Honestly, I'm a sucker for that.
But something I didn't know, I found out today. I was going around, joking around about my brother, calling him a jerk and normal sibling sh*t like that when he all of a sudden gets serious. I'm like "what's wrong" and he's like "your brother should be nice to you, at least he has a sister." I had no idea what he meant. Then he said "My sister died."
I felt bad. It kind of started an awkward silence, I didn't know what to say to that, especially since I met him the other day. All I could say was "Oh my gosh, I'm sorry." which is a complete understatement but what else was I supposed to say. I have no idea what happened, nor am I planning to ask, that would be rude on so many levels.
So I'm keeping him of course, since I'm grounded he can't come over so we're skyping today I think. I just didn't know what to say. I just wanted to get that out. Please comment with a little advice, it would mean a million! Thank you!