There are many expressions commonly used in English to express similarity.
There is no magic rule to help you remember them. You just have to learn them. Sorry!
Take a look at the examples below.
(To be) like one of the family.
This is used to describe a person (or animal) that is so liked and loved that they are treated as an actual member of the family.
E.g. We’ve known him for years. We treat him just like one of the family.
Like (two) peas in a pod.
This is used to describe people who are very similar either in appearance, behaviour or character.
E.g. I can’t believe they are not sisters. They are like two peas in a pod.
Have fun learning English!
Louise