Tim's diary charts the arrival of his mysterious Great Aunt Emma, an old lady who appears grinning on his doorstep and decides to stay. Nicknamed 'Grinny' for her constant eerie smile and mindless chatter, at first their aunt seems to be strange but harmless. But when Tim and his sister Beth make a horrifying discovery about their visitor, they realise that the safety of the entire planet is in jeopardy, and the silent invasion has already started.
https://www.amazon.co.uk/Grinny-You-Rem ... oks&sr=1-1
Just got this book today. Happily, it is one of those older, slightly obscure titles you can still collect without taking out a second mortgage!