What is the function of 哩? (Cantonese)
The standard character for ni1 / nei1 is 呢, not 哩, which mean 'mile'

呢兩個人好相似

呢 in this context functions as 'this'
呢 (this)
兩個人 (two persons)
好相似 (very alike)

呢 http://www.cantonese.sheik.co.uk/dictionary/characters/1029/
jyutping: ni1 / nei1
[粵][pron][adj] this

~

And how different is it if you just say 兩個人好相似?


呢兩個人好相似 (this two people are very alike)

兩個人好相似 (two people are very alike)


It is just like omitting 'this' in &quot;(this) two people are very alike&quot; in English.
You can omit '呢/this' if we know which two persons the sentence is referring to