Am i wrong for not trusting my cousin?

So i have this cousin who i grew up with, she lived with my mother and me through middle school all the way till high school. We were like best friends. but then i moved to another state with my now husband and she started hanging out with another family member. I moved back to my hometown state with my husband and found out that my cousin has been having relations with a family friend who at the time was my ex best friends boyfriend. I felt really bad because she would sleep with the guy and then hang out with his girlfriend like if nothing was going on. It disgusted me in so many ways, like how can she smile to her face knowing she's sleeping with her boyfriend? well because of her dishonesty i just don't trust her around my husband. Like one day my husband left his car parked in front of her house, he had went out with another friend but i didn't know until after i spoke to him later on in the day... but i asked her if she'd seen him because his car was parked in front of her apartment and she said no and got offended. Was i wrong to ask her. i mean am i wrong for not trusting her?