Is it possible both of us my brother and i share the same personal cloud over internet, and each of us won’t be able to access the other’s files and data?
Have you read the user manual for your My Cloud device? It has a section that details how to setup and use remote access / cloud access. And the user manual details how to configure Shares to Private to limit/restrict/grant access on a per individual user basis.
https://support-en.wd.com/app/products/product-detail/p/125
Also see the My Cloud Learning Center which has additional information:
https://learn.mycloud.com/default.aspx
This assumes one is using a My Cloud device and not a My Cloud Home device.
Does anyone still use a My Cloud remotely for sharing files ? … last time i tried it was slow as molasses
I ended up just using Google drive to share files with friends.
But, i still love using My Cloud on my LAN … no issues with that 
