Read this earlier and only now finding the time to respond (sorry, was at work and this deserves time)
! Before starting with what I have to say, I'll make it clear: my parents are divorced. This took place when I was young, and with time I have managed to get to know both my parents extensively and it's helped me realize a lot of things about the way families can function and the issues with these situations.
! The first thing I want to clarify is that it's not your job or responsibility what happens between your mom and your dad. Even though you are a family unit and there's a very strong sense of closeness implied in your post, the fact of the matter is that it's their relationship, and as a result whatever happens between them should be between them. Here's where that teacher of your sis made a good call by suggesting you and your sister sleep in a separate room … the arguments they have should never have been in front of you, mostly because then inevitably you two might wonder what you can do and how to keep the family together (more on that later.) That said, I want to clarify I'm not faulting your parents, as it's clearly a difficult situation and there are indications they are trying their best to keep everything in order. That thing the Korean lady said is quite correct, you should be thankful for having your parents, because all things considered they really sound like they're working extremely hard to put up an image of closeness in the family when their emotions may be completely different at this point. That takes a lot of strength, and it's clear they do it for the sake of you and your sisters.
! Now, on to your role in this. Again, it's not your job to keep the family together.
First, because the idea of a family getting separated is not that superficial. Divorces don't break families, people in families break families when they stop caring or talking to each other. In the case of a divorce, I can hardly imagine a situation in which you suddenly stop caring about your family. So no, I don't think your family will get separated, even if things might be strange and isolated at first.
Second, because their relationship does not equal the family. What goes on between them has nothing to do with their roles as mother and father, and it really only has to do with their roles as husband and wife. And in that sense, it's truly entirely their lives and decisions, and you should in no way be involved in this. Yes, they'll consider the impact their relationship will have on you, because parents would worry about that, but that's their consideration and not yours.
! And here I clarify why it's important to consider the long term, based on my personal experience: my family is from Latin America, very traditional culture, strongly against the idea of divorce, at least back in the day. My grandma never actually loved my grandpa and stayed with him only because they had kids and had to maintain "the family" no matter what. Nowadays, in their old age, they are both still holding deep grudges against each other and the resentment my grandma developed wound up emotionally scarring my mom and her sister, even to this day, because she inevitably wound up blaming her children for the fact she had to live an unhappy life with a man she didn't love. Staying with someone you don't love is a difficult thing, it can really lead to a lot of frustration because the fact is this person is holding you back from happiness. In this case, if they really are done with things, your parents would be keeping each other from actual happiness by staying together. And in the end, isn't their happiness more important? To establish a contrast, my mom having lived this led to her immediately divorcing my dad when it became clear she couldn't love him anymore given his actions. The whole family disowned her and I and left us to fend off on our own because of that … but in the end, everything was completely for the better. Through that divorce, my mother and I were able to travel, learn multiple languages, and become strong independent women capable of surviving in this new world. If it wasn't for the divorce, I would be nowhere close to being the individual I'm happy with being today. As for my dad, he still has his sadness and regret over losing my mom, but because of that divorce I was able to like him SOO much more. I am clearly aware of his faults and why it didn't work with my mom, but instead of resenting him for those faults, as I surely would had I grown up in a home in which he limited my mom's happiness, I instead get to see his good side and his likeable personality by interacting with him and seeing him as just a father and not in relation to my mom. Yes, the divorce thing sucks, I still find it weird that I don't know what it's like to grow up in a family unit ... when I visited my girlfriend's family it was downright bizarre seeing a family unit together and interacting happily. But I can't resent my parents for their decisions because our family would NOT have been happy given that they didn't love each other. And if there's no love, then what's the point of being under the same roof?
! I guess I'm rambling now, but, bottom line, what I feel is most urgent for you to realize, is that this situation is their personal issue and you and your sister should in no way interfere with their decision. OF course, be a part of the situation in the sense that you keep respect for both parents and support them in their very evident struggle ... maybe if your mom or dad need to talk to someone it would be interesting to do so (if the divorce is true, they are both suddenly losing their greatest friend and confidant … they could surely use that support!), but never with the feeling that it's your obligation or responsibility or duty to fix the relationship. Only they can do that, and only if both want to do so. And in the end it might also help you feel less of a weight on yourself as it frees you from such a strong responsibility (especially because if there's love the family will stay together regardless of whether they're in love or not because you and your sisters keep on existing and representing that family bond.)
edit: also, realize something else … if they notice that them getting separated is having a strong, negative impact on you and your sister, they might feel pressured to keep trying to make things work even when they don't. Letting them realize you support their decisions and want them to be happy might wind up relieving them of a HUGE concern and could lead to a faster resolution, whether it involves counseling (if they decide to keep trying) or divorce.
Perhaps instead of focusing on fixing their relationship, for example, you could focus on continuing to be a support to your sister who is younger and may have a stronger emotional and somatic response to the events. And focus on your life too! You're 15, you have the right to be studying and making friends and finding out who you are and where you're going. Don't let the problems of the older people interfere with your growth, I'm sure your parents wouldn't like that.
! Whatever happens, feel free to contact me if you have questions or need any help or advice. During the weekdays I work a lot but usually late at night I have time to be around and respond to things. Hope everything goes well and that my response isn't too late, I really wanted to respond earlier :\