[nuggien]

she seems to be blaming her mother for driving her sister towards depression and suicide. Kinda harsh. Could have been something else.

[mechanical_fish]

Not exactly. Depression killed her sister. She blames her mother for training her sister to hide that depression at all costs.

She may never know "why" the depression happened (as if there needs to be a reason) but unfortunately she probably has a pretty good idea of why her sister didn't turn to family for help.