She is stimming for a reason. Find out why.

Try to keep her buzy in activities which uses only her hands also put some stickers on her nails

Paint them, put nail stickers on them so he will want to keep them on?

Possibly get some good advice from your local chemist or even your GP.

following as my son bites his nails and the skin around them. he is autistic and if there is abit of skin sticking out he will pick and pick at it.

Have you tried the paint on stuff that tastes nasty? Or maybe get her to wear gloves? Offer rewards fir not biting tgem