help!

Guys, I need your help. About 5–6 months ago, WhatsApp introduced a feature for locked chats. I noticed this feature being used on my father’s phone around 2 months ago. I’m really confused about what to do. I don’t want my mom to get hurt, but I also can’t ignore this. I’m the eldest, and I usually don’t speak up or express my feelings to my parents. My mom already seems suspicious, and she’s even brought it up before. But whenever she questions my dad, he says things like, ‘I would never do something like that—I have daughters.’ Should I talk to my father directly, or should I tell my mom? Please give me suggestions. I’m feeling really stuck and don’t know what the right thing to do is.