Husband texting another girl

A few days ago I found out my husband was texting another girl, his sister in law. I wouldn't of cared BUT the texting was happening when we were apart during the day and even in the early mornings while he's getting ready for work and while I'm still sleeping, to top it off, he was deleting the messages. Once I found out, I was upset and confused so I confronted him about it, he says they were talking about his brother and drug addiction ( both his brother & sister in law are drug addicts trying to get clean, my husband has never been an addict though), he was giving her an ear to vent too and he says he was deleting them so I wouldn't think to much into the conversations. The texting has been going on for two weeks.

Anyway, I don't get how their topic of conversation can go on for two weeks, through out everyday while he and I are apart or hiding it, I can't get the thought that maybes there is more behind the texts out of my head and I don't want those thoughts, I want to believe him and trust him but the fact is, I can't see the messages to get the clarity that I need..

Am I thinking to much of it? Advice and opinions please?

**UPDATE**

Once I found out about the texting and confronted him about it I had this feeling that there was more to what he was telling me. On Saturday, I had a low point & lied to him and told him I found a way to get the messages back, he gave me his phone and I downloaded a silly app to make it look like I knew what I was doing. After that, he finally told me what really happened. He said there was a little silly flerting here and there such as telling her she's easy on the eyes and that she ended up sending him two pics of her in lingerie, yes, I was instantly pissed (I had proof on phone records that two pics were sent to him but none were sent to her). He told me other then that, nothing happened, he had no intention to stick his dick her in. Anyway, after all of this went down I finally feel at peace and can think again without the feeling that there is more. My gut and mind tells me that I can believe him about what he has told me. He said that he knew what it would look like from my point of view and he was afraid that'll I'll leave him, that's why he lied. I'm still upset that he lied to me from the start but like I said above, I no longer feel that there is more behind the story.

And about her, I texted her and told her that I don't appreciate her sending my husband pictures, she ignored me, of course but now she's knows that I know.