I'm sorry to hear about your sister.

I doubt there's really a "bipolar reasoning."

Think of it this way:
Have you ever been really angry? I mean so angry your heart pounds and your face flushes. What happens when:
1) A stranger walks up to you and says hi
2) Your sister walks up to you and says hi
3) Your boss walks up to you and says hi
Is your reaction going to be the same? Probably not.

Everything your sister's ever done to upset you is probably going to come to mind and you're liable to be hostile towards her.
How about the boss? Of the three, you're probably most likely to be the least hostile to the boss unless you're so upset you can't control yourself.

Would you be thinking logically? Possibly - but you'd still be motivated by factors other than logic (emotion).

Now magnify that scenario, extend the duration of the mood, and imagine it without a specific trigger. There is probably some degree of control, but it may be awash in the emotion. Mania/depression is like a lens which distorts every thought and action.

Of course, mania/depression is different from anger, but there are quite a few similarities.

I hope you don't let it get to you too much. I wish you the best of luck, and I hope everything works out!