YA novel about a young girl with baby twin brothers

The main character is a young girl (pre-teen I think) who has baby twin brothers, one is sick (his name was Clem if I remember rightly), there is an element of magic to the story, some type of spirit that she keeps in a jar or bottle that helps her baby brother get better. I have a feeling the twins might have been conjoined and then separated, but I'm not 100% sure on this. The book is definitely set in the UK.

I think I read this book when I was 11-13 so It wouldve been published around 2010-2013, the author actually came to my school and gave out signed copies but I dont have the book anymore and I cant remember their name. The cover was mainly grey with something blue on the front, I remember it being quite striking.

I've been googling like mad but can't find any matches whatsoever, it's really bugging me!