What was that story about teen cousins who get mental powers from polluted water?

Question

Anyone ever read a short story about these two cousins who, while visiting their grandma, fall into a polluted river and later find out they are getting mental abilities. Abilities to read others minds, partially influence others and build private worlds and barriers to protect their own minds. The main characters are a boy and a girl who didn't like each other at first, but get close, and have to work together to protect themselves when an older mean sibling becomes even more exposed to the waters than they do. Eventually figuring out the connection and vying for power. At its end it's a who will defeat who; the evil power hungry cousin who wants to control everyone around her or the young teens who eventually seek help from their mysterious grandma.

Key things I remember are: - the cousins weren't close in the beginning and it was their fighting that led to them going into the polluted water. -The older sibling/cousin (can't remember exactly what she was to them) was evil enough to kill or alude to killing her own parents/aunt/uncle. -grandma had some abilities too but how strong was unknown. Her mental world was a bit spider centric. -The two cousins at one point fight/test each other's strengths and worlds that ends with blending each world to resemble each other as to make it hard to know who's mind is who's when they are getting attacked. -The grandma teaches the two about "world layering"

I found it after one long night of just jumping from one short story site to another till I passed out in high school. Hate to think I dreamed it all up but any help is greatly appreciated. Thanks in advance.

Answer 1

Others See Us by William Sleator. Not a short story, but a short book.

Sixteen-year-old Jared falls into a toxic swamp just before the yearly summer reunion at Grandmother's house. He comes away with the ability to "visit" other people's minds, and things get very complicated very quickly. He discovers that his beautiful cousin Annalise is a sociopath, responsible for the death of another teenage girl, and that his ironfisted grandmother is an accomplished thief with even stronger mind-reading capabilities than his own. Grandma has her own theory about how to handle Annalise, and it includes exposure to the mind-expanding toxins. A confrontation of wills ensues, with Grandma ruthlessly bending others to her purpose.