Saw Tinder on my brother in law’s phone today

I was helping my brother in law with a scanned file on his phone today and I swiped down on the Home Screen, so it opened Spotlight (I think that’s what it’s called) which shows last/most used apps. The first one was Tinder. He didn’t react very nervously, but he did go back to the point on his phone where we started. I couldn’t really react, because we were in the same space as my sister, their child and our parents.

I am trying to be reasonable and not jump to conclusions. I asked my friends and we all did a test to see if it would’ve showed up in suggestions if he had just downloaded it, no decisive results.

I am not sure if they shared their phone codes with each other but my sister never has a problem telling me hers when I ask so I guess they also would share as a couple? Also, I’m 99% sure they share locations with each other, but I’ll try to somehow find out.

My sister is the breadwinner, she has a demanding job, but she absolutely makes time for family. My brother in law has his own business so he’s more flexible and is the one who takes care of their child between kindergarten pickup and her coming from work. I honestly don’t see a moment where he could cheat. He also just doesn’t seem like the type.

Should I tell my sister? Should I ask her if she knows why would he have the app on his phone? My friends also suggested I ask him about this, but I’m now sure about that. I have a week to decide as they are going on holiday (I found out the night before so I didn’t want to ruin it).

I need an outside perspective on this.