Kiely was pretty thin and also young, so I do think it is safe to assume to a degree that she would have gotten intoxicated fairly easily. I think we can trust she was drinking. I don't think she had Sprite when SS and the other friends were in the CRV drinking and charging phones.
It is a good point that toxicology would show more about what went on. Just how impaired she was, and what she may have consumed is not clear.
But we know she was drinking, and we know she unfortunately ended up in the water.