Bruiser by Neal Shusterman

When Brontë starts dating Brewster “Bruiser” Rawlins – the guy voted “Most Likely to Get the Death Penalty” her twin brother, Tennyson, isn’t surprised. But then strange things begin to occur. Tennyson and Brontë’s scrapes heal unnaturally fast, and cuts disappear before their eyes. What at first seems like their good fortune turns out to be more than they bargained for…much more.