TBF, I bite my fingernails down until there's barely anything left and my fingers are sore.. I have done since a child and so has my brother (his own fingernails, not mine
). With us it's nothing to do with how we were brought up - (we had a 'normal' happy upbringing with a 'normal' mum and dad!), I think I just have some kind of anxiety, may be genetic since we both do it. My sister never has though. I hate it as it looks so ugly! Wish I could stop. I managed to grow them for my wedding, but started again.
Could just be the same with Isabelle - it's just an awful habit. Doesn't necessarily mean she's stressed.