First of all, I want to say that you are very lucky. It is really lucky that someone can take care of you like an older sister and is willing to pay for you. From strangers to now, I believe that your relationship is no longer an ordinary roommate, but has become a very good friend. Then, you must seriously send a gift that carries your gratitude and your love for her.
Let me recommend some to you. Send her a plush toy. I heard you say that you live in the same dormitory, so you must still be studying. It is the blooming season. Sending her a plush toy is both simple and cute. If you think it is too common, then you can send her a glowing doll, which is both beautiful and special. Or you can customize one, which is different from some common gifts sold on the market. If it is customized, you can customize a real doll. You can also customize a crystal album and put the photos of the two of you in it. It feels very warm and touching. Or a DIY small gift also looks special and thoughtful.
Send the gift you prepared sincerely, and I believe your friendship will continue.