Solidarity. I pick at the skin around my fingers, just like the kid in L#2. My mum used to bat at my hands and hiss at me to make me stop.

I didn't know until I got diagnosed as autistic as an adult that skin-picking is a common autistic stim. Now that I know what I'm doing and why, I've been able to seek out stim toys that meet the same sort of sensory needs and don't make my fingers bleed at the same time.

If picking her fingers makes her feel better, it's obviously serving a purpose. Help her figure out what that purpose is and find her a wait to meet that need without doing herself damage.