I don't feel like I can let my guard down just because they're my family. They might be taking pictures of me without me even knowing about it.
ーI don't feel like I can let my guard down just because they're my family. They might be taking pictures of me without me even knowing about it/surreptitiously.
「家族だからといって安心できない。こっそり写真を撮られているかもしれない」
to let one's guard down で「気を許す・警戒心を解く」
without me even knowing about it で「私が知らないうちに・こっそり」
without me even knowing about it の代わりに surreptitiously を使っても良いですが、without me even knowing about it を使ったほうがもっと自然です。
ご参考まで！
カナダ 
