Flower Moon by Gina Linko

They are "mirror twins" and they even know what each other is thinking.  But things are getting strange and no one will explain...

Sky Pony Press and Edelweiss let me read this book for review (thank you).  It has already been published so you can grab a copy now.

They've always done everything together but now they seem to be separating.  Tally can't understand it and doesn't like it.  Tempest figures it out first but she's looking for a solution and isn't including her twin in the search.  There's something that happens to the twins in this family and they view it as a curse but they don't talk about it.

As time goes on, they find out that they act like magnets.  They are not attracting each other, they're pushing each other away.  They are also going through the change much sooner than other relatives.  They only have a number of days to figure it out.

The good news is that they are smart and they learn to use their powers together. This is a good choice for those who like fantasy.