Can other People across other devices view what I go on?

Currently, I share my ICloud and Apple ID with a sibling who also has an iPhone. I have VPN enabled and I was wondering if he can see what I go on regardless since our devices are linked

If he/she has your Apple ID, then they will be able to see everything that is synced with your iCloud account. Depending on your sync configuration and just what Apple's Cloud offers, then your Internet history and even open browser tabs may be synced onto their device(s).

Why are you sharing your Apple ID? Can they not get one of their own? Check out just what gets synced and what settings can be changed, I don't know what facilities iCloud offers as I use rival services myself.