Can someone explain why 们 is used in the word 哥们 to mean buddy/dude, even if it's referred to a single person? Would it be possible to add 们 to any other family name, like 妈们，姐们，爸爸们 etc.? And if possible, is it just a plural form or does it carry a different meaning like 哥们? Thanks!
Yes, 哥们 means "men" or "buddies" But you can still call a single person '你们哥们' (you men). Which refers him as "one of the men/ men like you". You are not technically addressing him, but addressing his group
For example, saying to a person: "You men are useless" suggest "you being one of them is also useless"
"你也是我的哥们儿了" means "you too are (one of) my buddies now"
If you have more than one older sister, you can refer them as 我家的姐们 (my older sisters).
Or you can refer a single female as 姐们 (one of the women/ women like her) similar to 哥们
You cannot say 我家的妈们 or 爸们 because you only have one mother and one father.
爷们 (males/men) can be used the same way as 哥们. As in "你们爷们" (you men)
Tang is right~
爷（爺）is complex than grandfather, it also meanings a man who has a higher opinion on himself or a lord. So 爷们 could be “lords” or “men” or even “a man with manliness”.
In ancient Chinese 爷 is also father, the lord of family.
们，is just not only plural, but also is related to relationship or relatives, like 姊妹们，姐妹们，妯娌们，兄弟们。
interpreting 们 as plural,is just what linguists or book of Chinese language regard as true, in fact, it is not natural and not true, both from view of historic Chinese and Mandarin.
们 adds sociality to these people，same for 姐们， blood relationship is not necessary。
妈们 would be an interesting word， if someone use this word， I would think he calls many women mom， these women give him money for some unhealthy exchanges， an horrible relation， same for 爸们。