Sister’s Fiancée’s Family - Advice Needed

Okay, I’m going to make this as short as possible but 6 years worth of information isn’t short. 😅

My sister started dating her fiancée a few years ago, they just got engaged a few months ago. We’ve had our fair share of concerns about him. He’s not loyal, not respectful, very awkward to the point of being uncomfortable for everyone around, and overall a very negative gut feeling. Anyways we had them over a few days ago; my boyfriend and I were smoking and having some general chit chat with him when all of a sudden we get on the topic of his background and what he’s on medicine for (we knew but not what or why). We found out he had a bad mental breakdown in college but he wouldn’t go past that - understandable. Fast forward to a few hours ago and we still can’t shake the alarming red flags. We find some of those background check websites and find that he has 6 charges against him and it turns out his parents aren’t exactly clean (sister has been admitted twice to mental hospital). Anyways, his background ranged from speeding/seat belt violations, DUIs, and assault and battery (class 3 I think it said). He’s going to marry my sister! His parents have burglary charges of someone’s home, both the same day. And here’s the kicker... dad is a pedophile and a child molester and was convicted THREE TIMES. I have a three month old son, of those three two of them were boys. So now protective younger sister and mama bear are feeling a whole bunch of things and I don’t know what to do. I need help on how to approach this with our parents (they still live with them) and my sister, and I don’t even know what she knows so I have to tread lightly. Not to mention I’d love to give his father a taste of karma. I need to tell them, as a new mother I do NOT feel comfortable. I do not feel comfortable knowing my sister’s fiancé is not beating her due to happy pills and if he stops taking them may as well have her end up there or worse. I have to worry if they break up will he come after her and do god knows what to her and/or my parents. I have to worry that if they end up having kids grandpa might be too involved with them, if you catch my meaning... I have to worry about my parents not killing him if this isn’t all exactly true but there are just too many coincidences for SOMETHING to not align properly. Any advice on how to approach this is much appreciated. I don’t want my sister hurt, and I’m so worried something will happen. My mind is in so many directions. And before anyone asks, my boyfriend’s best friend is an officer and did confirm to the majority. There were other things listed on the websites but we are suspecting someone else with the same name since it happened in areas they haven’t been (to our knowledge).

Thank you.

I’m posting anonymously because I don’t know if any of them use this app and would rather try and keep some sort of privacy. I know, this may sound fake but it unfortunately is my reality and I just want answers without losing my sister and dividing our friendship. Our cop friend gave us the response along the lines of keep away, there’s quite the laundry list wrong.

I feel so broken and my heart is so torn on what to do, but it needs to be done and said. I can only imagine what he went through as a child to be passed on to his present self and it honestly just explains SO MUCH about him and how he is. Sorry, I’m rambling.

Update: So we talked to my mom yesterday. She is going to talk to my dad, sister, and her fiancé separately. Due to the charges being out of state and also being “closed” online we can’t get very specific details or hell, even know if he knows most of what his dad did. So, not much of an update, but will keep updating as I know things. Keeping my fingers crossed my sister doesn’t go on the defense.