I think her knowing that MS was about to die was the breaking point which led her to take her life. There are likely a myriad of underlying reasons.
IMO, her last words were to JS, asking him to protect her sister.
so who exactly did she 'save' then, and like jenny asked, why would her sister need to be protected? why wouldn't she have written a note or letter or email to her family before she died to ask them to protect her younger sister?