For a nine year old, this is complicated. Goat's milk is milk, and though it's easier on the human body than cow's milk, it's still milk, so the question is why milk is being discontinued. A lactose intolerant person might very well still be lactose intolerant to goat's milk, for example, but someone who has digestive problems with cow's milk might not with goat's milk. There's also a taste issue. Very fresh goat's milk tastes pretty good, but still, it's not cow's milk. Soy milk is very high in protein, so that serves part of what milk gives you. Soy is a complete protein, as is cow's milk, but there can be allergy problems, so that can be considered. Personally, I drink soy milk with my cereal, but other than that I don't do milk of any kind, since it isn't necessary. Rice milk is low in protein, so it's a poor substitute, and is also very high in sugar. You also have to consider which soy milk to use -- some add sugar, some use more digestible sweeteners. Edensoy is about the cleanest, and adds seaweed for minerals. Almond milk is also quite good, as long as the added sugar isn't too high and the person doesn't have a tree nut allergy. And remember, none is necessary for good health, it's just what we're used to with cereal, but otherwise, there's nothing in any of them you can't get otherwise. There's also hemp milk, which will also have added sugar, but has a good amount of omega 3 fats and some protein.
I did a little research on Coconut milk simply because I have to stay away from saturated fats such that cow milk has. Turns out Coconut milk is a wonder drug for those that live in an area where saturated fats are nearly non-existent but has far more saturated fat than cows milk. I love coconut candy, cakes, and such which is what started the research. Turns out that Almound milk is one of the best milks out there for health issues. For your list, Soy has it's place but not for me, I've heard of Rice milk but don't know anything about it, and Goats milk isn't that far off from cow milk.
Basically what you should be looking at is your sisters health, what milk fits best for her health?
I recommend not giving soy! if you go to my journals and search for the topic on soy