My Dh started hanging out with distant cousin about 3 months as they were both were into a shared activity. I am not a jealous person and I have had no problem with their friendship. However another family member pulled me aside a month ago to say they thought it was odd and they had been discussing it with my SIS and they both agreed it seemed fishy.
I have never ever done this but I looked at his phone a month ago and saw they were in fact texting each other a lot but it seemed completely innocent. And when cousin was over, she spent more time with me than talking to him and they would laugh openly about the stuff they were doing on their shared activity so thought nothing more of it.
However, DH has been very grumpy with me the last 2 weeks and I have naughtily looked at his phone again. They are texting every day - Lots of comments from DH about how wonderfully she is bringing up her children, her saying she is on a diet, him saying she doesn't need to be. And what is really upsetting, dh is now talking about me, asking her advice on our relationship and she is giving neutral advice but talking about a private girl's conversation we had recently, saying some other friends were laughing and condoning my selfish behaviour which is not on. He actually asked her if he could sleep on her sofa this month if things go tits up between me and him. The only thing he has said to me is that he would like us to go a couple of nights out this month just me and him.