Should I be worried?

The other day my husband said to me to quickly txt his friend back with a time of when they are meeting up. So I grabbed his phone and went into WhatsApp and found his friend and txt him. We don’t hide our phones from each other and we don’t go through each other’s phones. But as I was in WhatsApp I noticed a name I do not like! It’s his best friends sister. They pretty much all grew up together and he’s told me he has never done anything with her. Me and my husband have been together for 12 years. I know his friends very well and consider them my friends too. But this girl I do not like. I got a bad vibe the first time I ever met her. But at the time all those years ago my husband told me he sees her like a sister. Which I could understand. But I had seen txts years ago where they were kind of talking about me. Nothing too bad but it pissed me off and I have never liked her since. But curiosity got the better of me and I had to see what they were txtn each other. I did not know they were still in touch. She moved away a couple years ago which meant she wasn’t at the wedding which I was delighted about. So now the txts I came across we’re very recent. They pretty much txt every single day. Nothing really bad or that. But they had met up!! They went for coffee couple months ago and I didn’t know anything about it! And they had both wrote about how good one another was looking. I was extremely annoyed by this. I haven’t said anything to him because I don’t want him to think I was snooping through his phone, even though I kind of was. I just get a really bad feeling about it. And I haven’t slept properly in days.