I think my fiancé is sleeping with her brother

I’m not even sure where to begin this honestly this has been on my mind for a few months now I believe my fiancé [22] is sleeping with her brother [19] . Me and my partner have been together for 4 years and have recently engaged ., many things have lead me to believe she’s having an inappropriate relationship with her brother when I went home for Christmas of last year I found condoms in the trash can when I confronted her about it she said they were her brothers and he recently had girls over when their parents were out of town . The next thing that added to my belief was when we were gaming and I heard playful laughter and spanking in the background and her jokingly saying “stop” they also tend to always spend time together I mean they practically everything together there’s time to where she’ll wait for him to get home from work to run a simple errand like going to get coffee . I finally had the confidence to ask her if she’s having an inappropriate relationship with her brother or not and she got extremely defensive and started calling me weird for having such thoughts. Usually I can piece together what makes sense and what doesn’t but this has been on my mind for months and I’m constantly questioning if it’s true or not it’s just getting bad for the mental health and I want to make sure I’m not crazy . I mean when we first started dating their relationship was a bit toxic it seemed as if they didn’t get along well but ever since I left for the military last summer I’ve noticed their relationship has grew closer maybe it’s just a normal sibling bond and I’m reaching?