You might use the cliched phrase,
(as) alike as (two) peas in a pod
phr. very similar. (The peas in a pod are essentially identical.) The [sisters] are as alike as ... peas in a pod.
(McGraw-Hill's Dictionary of American Slang and Colloquial Expressions. S.v. "as alike as two peas in a pod." Retrieved December 18 2015 from http://idioms.thefreedictionary.com/as+alike+as+two+peas+in+a+pod .)
Usually, but not always, the phrase may specify two peas. It fits as well with three peas, or generally, without mention of how many peas are involved:
Anne, Emily, and Charlotte Brontë are such talented writers. They are as alike as peas in a pod.
I've heard the phrase used alone also, relying on its familiarity and the context to fill in any omitted sense:
"They're very alike, aren't they?"
"Peas in a pod," he replied.