The big point of parentification is that it is forced on the child by their parents. The role of sub-parent to siblings is assigned via negative means. This is often in punishment for not performing that role. This can manifest as verbal and mental abuse, such as guilt tripping, neglect where the parent doesn't do what is needed until the parentifide child does it out of necessity, and can go as far as full blown violence.

This reads more like the OP saw that their sister was in desperate need of help, and decided to take it upon themselves to do whatever they can to help her.