What is the function of 哩 in 哩兩個人好相似?
And how different is it if you just say 兩個人好相似?
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 "(this) two people are very alike" in English.
You can omit '呢/this' if we know which two persons the sentence is referring to