Shinta is right. Kraco, your reasoning would work if Ranka knew what the outcome/influence of the varja would be if she sang while being upset/distraught/however you want to put it. At that point, everyone just thinks that Ranka singing = varja cool down. So it's only natural that they want her to sing even if she didn't feel like it.

We (the audience) are fortunate because we get to see the cause and affect. Kathy and Osma have no idea that it's Ranka's singing that caused the escalation. They just got to see the varja get angrier. Whether everyone gets together to compare notes and figure out that negative emotion singing Ranka = angrier varja we'll have to wait and see.