Lost some of my photos / mine appearing on brothers phone

Out of all the endless irritating annoying unexpected behaviour of iOS 18 and the fact I’ve used iPhones for over 10 yrs, for the life of me I cannot understand how I can lose photos and the same ones appear on my brother‘s iphone instead. 

I’ve read countless articles trying to explain this behaviour but nothing matches my scenario which is simply that we have two separate distinct Apple IDs and all I did a week ago was make a shared album and invite him through his iCloud ID. That shared album appears on both our phones. I’ve deleted it now in an attempt to stop this nonsense of my photos disappearing and appearing on his phone. 

My next phone will be a Samsung. 

Any ideas much appreciated. Thanks.