I remember reading this book back in 5th or 6th grade (I'm in 9th now), about a girl who moves into a new house with her mom and sister (maybe dad too; I can't remember); but her sister goes missing, and this cat gives her powers to turn into a cat, I think. It goes through a lot about her figuring out how to use her powers, and then she has to go save her sister.
This sounds a bit like Claws, a 2012 YA novel by by Mike and Rachel Grinti.
The blurb on their site reads:
In a contemporary fairytale as irresistible as catnip, one girl discovers that some magic cuts deep...
Emma's sister is missing. Her parents have spent all their savings on the search. And now the family has no choice but to live in a ramshackle trailer park on the edge of the forest, next door to down-and-out harpies, hags, and trolls. Emma wonders if she'll ever see Helena, and if she'll ever feel happy, again.
Then she makes a friend.
A smooth-talking, dirty-furred cat named Jack. He's got a razor-sharp plan to rescue Emma's sister. He just wants one small favor in return...
I haven't read it myself, but have scanned through it when my eldest read it. If I recall correctly, she doesn't turn into a cat, but rather gains the ability to control them.
Another thing noticeable is that the book contains excerpts from a site called CragWiki.org; a sort of Wikipedia of magic.
It has over 90 reviews on Goodreads, most favourable.