There is a lot going on here.
If she is able to refuse to take her meds, her fever is not that bad.
The simple thing to do is remind your daughter NOBODY likes to take medicine.
Since she will not take her medicine, she will need to be on total bed rest. This means stay in her room, stay in her bed with one book. No music, no TV, no toys, no open curtains or blinds, no siblings (they could catch this flu again! Wink, wink).
No begging, No bribing, no threats. Do not make a big deal.
If she needs to go to the potty, tell her you need to escort her and stand there to see what she passes into the potty so that you “can report back to the doctors office how much fluid she is passing”. Stand in the bathroom with her, do not make a big deal.
Children that take their meds, get to rest on the sofa or a palate in the TV room and watch videos, get to have jello, even sip 7up. Those in bed in their room will get basic liquids.
This second child wants control, wants attention and thinks that because she is the second one sick, she missed out on all of the attention the first child got when she was first sick with the flu. Hang in there mom, I am sure you are exhausted, just stand firm and do not over react.
FYI. The old fashioned way to get rid of a fever? Cool clear liquids and cool temperature baths.