Thanks for the info and prayers/vibes everyone.
I talked to my sister last night and she said it is only in in one lung, and there was so much fluid there that it basically collapsed that lung! They gave her a whole battery of tests yesterday and took some of the fluids out, which she said was highly painful!
They will also be checking the fluid to make sure there's no cancer there, but the doctor doesn't think there is because of the way it looks to the eye.
She did have congestive heart failure about 15 years ago when she got a staph infection that build up fluids around her heart. But they haven't said any of her heart tests were abnormal so far.
Thanks again everyone, I'll keep you updated.