Acute (Medicine).

1) Having a rapid onset and following a short but severe course: acute disease.

2) Afflicted by a disease exhibiting a rapid onset followed by a short, severe course: acute patients.

Given the preceeding definition, I would think it means that your sister is in the midst of an episode, probably mania, as that's when pdocs tend to diagnose bipolar.

The best thing you can do for her is to be there when she needs you, and and to realize that she may do or things you perceive as harmful or hurtful that she wouldn't say or mean when she is "normal" (stable).

So, the best thing for now is to support her as best you can...I wish you well.