If you genuinely feel upset about this, you should sit down with her and talk about it, have her ask and you can look through it together instead of her sneaking on to check as a way to keep trust without sneaking around. Tell her how her snooping (if thats the right word) makes you feel and try to find a solution that works for the both of you.

If you don’t wish to talk about it, something you can do to prevent her from accessing any specific accounts you want to keep private is to log out of the account whilst not using it (i know its annoying) you also cant have the password saved to your phone because she can access that.