Ummm like everyone else my knee-jerk reaction is to say stay away. But come on, it's your sister's 40th, you guys planned a trip, so this is obviously a big deal and you will feel like you missed out on memories if you don't go. If it was me, I'd probably find a way to make it work. Some things I would try:
See if you can figure out what she really has-a lot of people without lung issues confuse "pneumonia" with "bad/persistent cough." The antibiotics might clear up whatever she has before you leave. If you feel comfortable asking, find out what antibiotics she is on and ask your doc for a prophylactic prescription for you that would give you coverage against similar bugs (e.g. cipro or bactrim depending on if she's growing something gram-negative or positive) to take with you. If you don't feel comfortable, anything wide-spectrum would probably work.
Can you travel in different cars or sit far apart on the plane?
Stay in different hotel rooms?
If you guys break into groups, might you actually have very little face time with this girl?
Don't sit next to her at restaurants, etc.
And finally bring a mask and antibacterial gel.
If in the end it's too risky, if she has some weird virus or is really really sick (in which case it's dumb of her to be traveling), I guess you would have to stay away. Also, if you are not at 100%, I think you should stay away. But if this is a big deal, I would try to work something out.