Being in a member of the Weasley-Potter clan can be intimedating, espesially if you're a middle child. Especially if you were Lily Potter.
Lily doesn't have the confidence her James. She's not good at Quiditch like Fred and Albus and she isn't an amazingly talented witch like Rose and Lucy. She isn't particularly pretty and doesn't have the ability to charm anyone she wants, like Domonique and Hugo can. She doesn't have plans to live life without the status quo like her muggle loving cousin, Molly. And she isn't the first to everything like Victorie and Teddy.
Lily is just plain Lily...
Until the year Albus started at Hogwarts. Letters were owled to and frow, and despite being the only child in the house, Lily still never did anything worth while.
So that christmas Lily stops talking.
Now in her fifth year at Hogwarts, Lily still hasn't uttered a word...until she meets Cole...