How about I keep reading my sisters diary while she is at work.
Why did you read your older sister's diary when she's at work?
I just wanted to see if she's faithful, but I'll hold my tongue on that...
Snooping through your sister's diary is a violation of privacy and trust that will cause her to lose her trust in you. If you have doubts about her, sit down and talk to her since communication is they key to a successful relationship.
Is it really important not to peek into her diary to keep the peace and family unity?
It is totally wrong, you wouldn't want her to do the same thing to you right? Don't you think she might get really angry and yell if she finds out you keep doing this frequently?