skin sore on chicken's belly - what is it and how to treat?

[NewFlockOnTheBlock]

Update: breast blister looks better, smaller and less inflamed. We are continuing with Vetericyn and Neosporin twice/daily. Prefix went broody a few days ago, which is both good because she is less likely to spend time picking at the sore, and bad because it rubs against the nesting material all day.

I do have a question related to those swollen joints in her ankles: is there any treatment for it? Even just symptom relief? I worry that it causes her pain.