Everything happens all at once

I'm a month into my job and am starting to wonder a bit if this is all there is (short answer, unlikely, but I'm impatient), my marriage is on the rollercoaster big time, and...so many things to juggle and worry about.
Now, add one more. My baby girl has mono.
Mira has been sick since last Tuesday, a bit over a week, now. At first, the diagnosis was strep, mostly based on the fact that Soren also got sick and had a positive strep test. Mira had the same symptoms but was worse than Soren, and her tonsils looked shredded. Our doc didn't want to swab them, and all signs pointed to strep in Mira as well. When the antibiotics didn't help, however, our doctor moved both kids to a new prescription and then ordered bloodwork to see what was going on underneath. Answer: Soren did have strep, and is now fine. Mira may well have had strep too, but her main problem is mono.
In the past week Mira has lost a pound. She went from 53.5 to 52.5 pounds. Mira is 10, and has hovered around the 50 pound mark for about two years. We've been hot on the trail of what is causing her to be so skinny for some time now. Considering that Chiara, at age 6, weighs 47 pounds (and is a head shorter than Mira) and Soren, at age 12 and about an inch or so taller than Mira, weighs 75 pounds...I have a sense that Mira is about 15 pounds underweight, at least.
So, how to get her better and also help her gain weight. The saga will continue....