This often can be a generalization, Bipolar is not a "one size fits all", I'm not replying with resentment, but feel people need to stop holding onto stereotypes, maybe even educate themselves using professional info. My brother was DXed BP-I years ago and yes when not taking meds properly can lose and has lost control in the past, lashing out saying horrible things, once stabilized on his meds and sticking to them, he is the loving brother/friend as he always was. I was DXed with BP-II, and I have never had that happen to me, instead,I'd withdraw into deep depression. Not all forms of BP are the same.
No one is the same, as well as no Bipolar is the same, despite some sharing similarities. The usage of "Bipolar" has become overused and abused by many people, for example:
if a person has been spending money, buying stuff, etc. automatically the label "he or she is bipolar", if someone is terse, "they are being manic". Seems like "bipolar" is now replacing "add/adhd", like when someone would be energetic or fidgety, we use to hear catch phrases, "oh he or she is hyper", "or so and so needs their ritilin", etc.
Maybe I'm just rambling here . . .
But so often, Bipolar is misused to describe people with unwanted personalities, when in all actuality are not Bipolar,or Bipolar combined with other disorders, for people can have multiple disorders too. It may not be Bipolar at all, but they may have BPD,and/or just grew up in an environment conducive to the person's behaviour and/or personality. I have seen times when my brother would be so wound up, in a full manic episode, and then calm for awhile then start up. I truly believe him when he has told me what it is like and how a person DOES NOT have control, he use to even cry when an episode would be approaching, telling us how he can't "stop it", and that is where the proper mood stabilizer(s) came in, and "rescued" him and us, as well. Sure, like anyone with or without this disorder may use their lack of control for an excuse, but I doubt anyone truly Bipolar wants to be out of control, and I highly doubt from what I've seen, that someone who is episodic can just volunteerily (sp?) turn it on and off to their convenience, it's more complex than that.
Maybe I'm not being much use to answering your question(s), and I even apologize in advance, if I'm way off on them, I'm not saying your sister's behaviour is not of bipolar behaviour or not, I do want to say that of what I have witnessed with my brother, is that while he was manic, there is no reasoning with him, sooooooooo we had to refrain from any trying to reasoning, which we learned from one of the top pdocs in the eastern part of the US, as well as several other pdocs.
If your sister is not on mood stabilizers and/or not in some treatment plan, chances are she will continue this, but there are people too, without a Bipolar Dx that can behave like this, so this is why it is always best for the person to have a professional psychiatric evaluation, and then work with the pdoc for treatment/therapy.
I wish you luck with this, and that your sister gets professional care.
Take care,
DE