Dorsal fins curl usually due to unfavorable water conditions, whether be hard water or unclean water, and could also be genetics. I have two who have a big curl with thinning on their dorsal fins, got them that way. There is no way to treat it (they are in the best water conditions, as well as one was in AQ treatment for 10 days for a different reason). Just keep up on the water changes so it doesn't get worse and feed a variety diet high in protein. He's healthy, it's not a disease, just an abnormality.
I have a ST, very pretty fin shape :) And the coloration you see is on him is called cellophane. My ST has the same thing- he's dark red/blue/white with a metallic blue cellophane on his fins. Hope you get a picture up so we can see your sister's pretty boy :)