Sister’s Instagram?

So I looked on my husband’s computer history and he’s been going to my sister’s Instagram almost every 3 days... I confronted him about it and he said it was because he searched for mine on Google and it didn’t pop up so he found hers and accesses mine through a pic I’m tagged in...

Not sure I believe him. My sister has some sorta sexy pics on there. He was able to name off captions of photos on my Instagram to prove that’s where he goes, but it still seems weird to me. It’s been going on for months. And once he found mine, why didn’t he just memorize my username and search that from then on instead of continually searching my sisters? I’m so sick about this. We have a 17 month and 2 month old and my sister and I are close and now I’ll feel weird if they’re around each other.

Would you believe him? If not, what would you do?

Update to Answer Questions:

I checked his history because we both agreed we didn’t want each other watching porn, and I found out he had been

And he doesn’t have social media. My sisters Instagram and mine just aren’t private.

I just don’t know how to respond to this at all other than being pissed... my sis lives 10 min away and my family gets together a lot so she’s unavoidable