I was in this same situation last year.
I had to put my boy down at 2 weeks shy of his 8th birthday. The days leading up to putting him down he became very sick. Our female, 9 years old (They've been together his whole life), could tell he was sick. She was very depressed. Finally I knew I had to put my dog down. The female was very depressed after, and she had to be put down 6 months later. It was extremely sad. The only consolation I have is that they're together now. I don't think she wanted to be here without her brother and best friend.
I'm very sorry to hear what you're going through. It's the hardest thing I've ever dealt with. I'm sorry and I hope the best for your girl, and I give my condolences.

Oddly enough both of our dogs died of the same thing. Idiopathic pericardial effusion (the sac around the heart filled with fluid and also the abdominal/chest cavity). Ran every test in the book and nothing was found. It's nothing that can be caused environmentally either