Mum and dad first, word with them and then confront your sister about this.
Expect massive anger from her, blaming you and your mum and dad, no acceptance she is to blame....then tears and threats to take her own life. There could also be an incident where she 'has a go' but won't be very good at it...oddly.
Never had this, seen it many times with other family members and friends. She is, purely based on your post here unable to cope and potentially mentally unstable. What she might also be and I suspect more likely to be is sub human scum/psychopath with little or no sense of responsibility. She will never own her problems, they are always someone else's. This tends not to be the way stressed or depressed people act, it is how scummy people act.
My advice is simple. Protect mum and dad, be firm, very firm with your sister and prepare for her to act like a proper mental....but protect mum and dad and she will either see sense (great), get worse (**** her), try to take her own life badly (remain firm, protect mum and dad) or take her own life, sad but not your fault.
There comes a time when you can do no more. Try but if all else fails protect mum and dad and get on with you own life. Cruel to be kind, I suspect she will take some sick pleasure from involving others in her charade.
