I just want just to make sure you have a clear understanding when continuing to make your plans. You mentioned earlier something along the lines of your sister offering to pay for fees, the plane ticket, and expenses minus a month’s worth of rent, so how could you say no. I just want emphasize that she really can’t actually offer you these things. You are not her dependent, so even when she turns 20 and is seen as a legal adult in Japan she cannot bring you into the country. All the visas and plans for the two of you will be independent. That being said you can try to apply for the visas/ scholarships offered that have been mentioned by others, but you also cannot bring her into the country. Also, with you still being a minor I’m fairly certain your mother will have to be involved with all of that paper work (Someone please correct me if I am wrong) since your sister has no legal rights to do so. This will depend though if the paperwork you need to fill out goes by the Japanese adult age or American adult age, if it’s the American adult age then you should be able to fill it out once 18, but be sure to check on this. As for your sister, from what I can tell she probably cannot get a teaching visa or a work visa, so she may have to compete with you for the same types of student visas (unless you both postpone things for the 2+ years needed for her to meet some requirements for other visas) You seem pretty set on trying to make this work and I wish you luck. I just wanted to make sure it was clear that she cannot bring you, you have to bring yourself.
Also, for renting many places require 1-2 months rent on top of other cost upfront. It is not a deposit, but something specific to renting in Japan, so thousands of dollars could be needed up front to get a place. This is something to keep in mind when planning finances. You’ll also need to hire a company to find a place for you and also hire a guarantor. If you come on certain visas/ programs they may take care of housing for you, but probably wouldn’t allow another occupant nor pets.
Once again, you seem pretty set and I hope you end up in an improved situation soon. This was mainly meant to be an information drop, so you don’t continue making plans with possible misunderstandings or lack of information.