Bwahahahaha, there’s a Shanghaiese in the woodpile, boy do you want to live on the edge. (I married a gal from Shanghai 15 years ago 
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Chinese New Year’s eve is only for immediate family, so I’m guessing you two are beyond the casual stage. If you don’t think you are, this is a sign that your love muffin thinks you are. You should probably clarify if that’s a grey area for ya.
Ok, main thing, are there going to be any relative’s kid’s there like nieces, nephews, grandkids, etc? For them, you want the “lucky red pocket money.” Find out and then we can address it.
Then localize. Win over the future mother in law with some shaghaiese. Get your girlfriend to teach you to say the Gongxifacai (恭喜发财) in Shanghaiese. Sounds something like “gong”, “she” like a girl, fa like father, and zey.
Find out if your future father in law is Chiuchow, Shantou or plain Cantonese. Again, they will all have a different varient on Gongxifacai (恭喜发财). You may have heard Kung Hey Fat Choy, which is Cantonese. But Pops might be a different dialect speaker. Knowing how to say this phrase in his dialect is good brownie points.
Ask your squeeze if her parents like a little humor? When you whip about the above dialectially correct phrase, the parents will say it back to you. Now for the fun part, you can answer back with “hongbaonalai (红包拿来)” aka a rhyme that means “gimme the lucky red pocket money.” Not pulling your leg. For a gweiloh talking dog trick, this one is a crowd pleaser (unless you’re hanging out with some pretty stiff Chinese). And, honestly, this whole crapola about “inscrutible Chinese” is a crock. Most love an unexpected corny rhyming joke at New Year’s. But double check with your love dumpling first to make sure it’s kosher.
Bring a decent bottle of booze. Doesn’t matter if it’s wine, scotch, brandy. However, if you go for the hardstuff, it needs to be a “name brand” and even better if in a tacky bottle. Seriously, brandy for example needs to be XO, Remy Martin or some other overpriced fou fou stuff that can proudly sit on the shelf and scream nuveau riche. Don’t get suckered into buying something better but obscure. If you happen to pass through a Duty Free shop before then, they can help you out as almost anything Duty Free carries by definition would fit the bill.
And then eat with gusto. Remember, in the Chinese culture, a good satisfying belch means “great meal” rather than a faux pas. Of course, being kinda Canadian Chinese, one might want err on the side of a discrete and possibly barely audible belch with no shame. Oh man, the food is great. Try it all, even the jellyfish and fermented winter melon. I like 90% of it and usually it’s a ton of dishes and you can start with a small bite of each. Hint: if one dish is gnarly, just swallow the morsel, toast the host with a big old honking “ganbei” / bottoms up and then tuck into something you like.
The only maybe wierd type thing is if they have a family alter. New Year’s Eve you would light some joss/incense sticks, bow a couple of times, think good buddhist thoughts and place in the bowl. Again . check with your girlfriend if this is something they do or not. If they do, then maybe practice first. If not, then fugheaboutit.
feel free to come back with more questions.
Definately don’t wear black - it’s associated with death and executions and definately isn’t a holiday color. Red is good and the perfect time to whip out that cheesy red sweater from the back of your dresser.
