I'm not really sure where your mother got the idea that, specifically in China, wasting food is a sin. I mean, sure, it's generally frowned upon to just needlessly waste it, but if you can't eat something, then no one is going to comment on you for it. I always thought that not wasting food is something that isn't necessarily limited to the Chinese culture, but I could be wrong. Don't let her make you feel guilty about it. You weren't comfortable eating it and therefore, you shouldn't have to. Besides, if everyone there knew you are a vegetarian, then they shouldn't mind you taking more than your share of the veggie dish. The fact that your mother tried to interpret Chinese customs to make you feel guilty is a little shocking. Was she trying to blame the culture or something?
I can't help you with the vegetarian at a family style meal part, but I can lend a few FYI's about Chinese food:
1) Vegetarianism is usually somewhat of a rare thing in Chinese culture (I've only known it associated with monks), so most of the old-school, authentic restaurants won't have a lot of dishes that accomodate it.
Corallary to #1: Faux-meat vegetarian dishes are popular in certain parts of China and I know of several places that do soy-based faux-meat and all veggie products. Ask your uncle's boyfriend if he's heard of any.
2) Even if the dish is veggie, there might be animal products, like animal fats or animal-based broths used to cook it. You won't likely get someone to tell you what's in what at a mom-and-pop place. Stick to the most basic stir-frys you can find. Look for things like garlic sauce (usually garlic and a little water with cornstarch as a thickener) and soy sauce. Don't expect them to lay out every single ingredient for you. Trust me, I've seen people attempt it and they only end up really frustrated. The only "Chinese" places you should try that at are the Westernized ones, like Panda Express and P.F. Chang's (do they have either of those in Canada?). If language is an issue and if your uncle's BF speaks Chinese, then have him ask, but don't expect a detailed list, by any means.
3) If all else fails and you're hungry, eat rice. It'll tide you over until you leave and can get something else. Or eat a small meal beforehand.
4) Don't listen to what your mom said. You don't need to put your dietary requirements aside for anyone, but a little more understanding on both sides can help a lot. I would definitely explain what the limitations are very clearly to your uncle, if not his boyfriend, so that they can always make sure that you're accomodated for. My BF isn't Chinese and my parents have been accomodating him at any big super-authentic meal that we have. He's not a vegetarian, but likes the Westernized stuff and is a fairly picky eater.
5) Don't feel bad about the red bean soup. I can't stand the stuff and I grew up with it. Heck, my mom would have made me feel guilty if I DIDN'T give that to her, because she always welcomes a good red bean soup. In the future, if you have a choice for dessert, I highly recommend almond jello. It's fantastic and completely vegetarian-friendly.
6) What blarg314
said is right. Chinese people tend to be solicitious to the point of paranoia when it comes to feeding people. It's tradition to make sure everyone is fed to the point of near-explosion, so making your needs and preferences clear would probably work very well in your favor. At least, at my house, it would.
This last part is just my opinion, but I'm American-bred and have been in the country for 23 years. I love country music, make one heck of an apple pie, and speak flawless English with a California accent. No matter how American I am, I will never learn to love the Westernized versions of Chinese food. I enjoy eating the authentic stuff and am lucky to live in an area where it's available. I would be incredibly hurt (not to mention offended) if people, especially my BF's family, though of that as "chinky". With all due respect, it is not funny at all.