I feel like something is going on but he is denying it

I found messages between my bf of 6 years whom I live with and his sister in law. She’s been separated from my bf’s brother for some time now who at one time brought up how he thinks she flirts with him (but he’s crazy jealous so I brushed it off, until now) some of the messages were casual chit chat but then I seen good mornings back and forth and messages from her asking what happened last night did he get in trouble for being on the phone or oh I didn’t want to get you in trouble kinda stuff. I obviously confronted him about it and he flipped it around and got super pissed saying I’m crazy and nasty for thinking he would be doing something with his brothers ex wife and his niece and nephews mom. But why the hell would she be asking what happened last night and joking he would get in trouble if it’s innocent talking, I’m so hurt right now and have literally no one to turn and talk to for advice. Worse thing is it’s my step sons birthday party tomorrow and she’s going to be there I want to confront he so bad just like I did him and worried it might come out outta anger tomorrow. Am I wrong for feeling this way and blowing things out of per portion like he says or do I have every right to be thinking he’s lying to me