Yes it is a mental health disorder
It hurts to self harm, people do that when the pain inside gets overwhelming. Your Dsis is reaching out to you
You may wish she dealt with things better, more constructively but she can't and she's telling you she can't. Because she doesn't have the emotional resources to. Try to get her to talk to her GP . Talk to a professional yourself, even if it's advice from a local charity for mental health (like MIND)
It is terrible times right now and the incidences of Eating disorders and self harm are increasing but mostly unseen as no one is seen much as people are all behind our lockdown doors. Please encourage your sister to seek help. It'll be too much for you to take on as a secret only you know.
Does she have DC? Is she alone?