“Because they don’t like you”

This, 100%.

If I were in your position, I wouldn't marry him.

Red flag #1: he tells his family about your fights. It is unlikely this habit will go away. With serious discussion and counseling, he may be able to understand why this is a toxic habit, but the moment he gets into a heated situation with you, this will go out the window and he'll "vent" to his mom and sister again.

Red flag #2: he has used his own poisoning of his family as the reason he didn't propose.

Red flag #3: you had to have a discussion to convince him to propose to you.

It WILL be crazy awkward. My in-laws hate me (due to religious belief differences), the feelings are mutual (they are terrible people and I wouldn't want them to like me anyway), and whenever I have to interact with them, I am uncomfortable and the hatred is palpable. I generally find excuses to avoid them entirely, and in the last 6-7 years I have seen/spoken to them maybe twice. But I never had to convince my husband to marry me, and he doesn't care at all what his family thinks of me - and he doesn't talk to them about me or our relationship, because his own relationship with them is not good, either.

Take this advice as you will. But you should know that you deserve to marry someone who has zero hesitations about marrying you, and you deserve in-laws whose opinion of you hasn't been poisoned.

I tell my mom and my friends about fights my fiancé and I have, that fact doesn't mean in itself that your SO isn't worth marrying. The part that is problematic is that because he has gone to them, his family has made their assumption of who you are based on your fights, which isn't fair to you or your bf. I think at this point, the best thing that could happen is your bf having a heart to heart with his family and letting them know that what he said was all in the moment and that he let it get too far by telling them everything in that light.

I tell my mom all sorts of stuff, but I make sure to occasionally throw in that I'm just venting and that things truly aren't bad. I work my thoughts out by talking, so normally by the end of our conversation I'll be thinking totally different stuff than when it started. This may have been what he was missing. He meant to just get some stuff off of his chest, and they ran with it.

I'll say as well, my MIL HATED me when my fiancé and I started dating, I even walked into the room as she was trying to tell him to get rid of me. You don't have to have an amazing relationship with his family - you can suffice with a cordial, polite relationship if you're both okay with that. At this point, if his family isn't ready to invite you in with open arms, just do what you can to keep things light and let him deal with them. His talking started this, so he should be the one to make the moves to reconcile, not you.

I had a very similar situation with my first true boyfriend. (I had dated people before, but this was the first person I was in a serious long-term relationship with. At the start of the relationship I was 19 and he was 22.) We had started dating in September; he spent Thanksgiving with my family and I spent Christmas with his. When I was there everything was good and they really seemed to like me. And they legitimately did. The boyfriend and I didn't really fight, but we had small disagreements and I wasn't shy about letting him know when something was bothering me. However, every single time we had even the smallest of disagreements or I was bothered by something he would call his mom and talk to her about it.

Not once did he ever call to say that things were going great with us or talk about something nice I had done. (Not to toot my own horn, but I put on a massive surprise party for him for his 23rd birthday that included a ton of our friends from the line dancing community. I had given a friend a key to my house so the friends could gather there while I took him out to dinner and then surprise him when we got back to my house. He has a TON of food allergies and I specifically baked a regular cake for guests and an allergy-friendly cake for him that was actually really good. He cried because of how happy he was and literally spent months thanking me for doing that for him. But did he even mention this to anyone in his family? Nope.)

Since his family lived 3 hours away and he spoke to them on the phone when I wasn't there, I had no idea that their perception of me had changed. A few months after his birthday his parents and sister came down to visit. He asked me to sit out dinner with him so he could spend some time with just his family because he didn't get to see them in person very often (cool, I get it, that's fine). Then I met up with him to go to line dancing and his family met us there. Things were..... very strange. I tried talking to them and they were cold and distant, only giving single word answers and making no effort to hold a conversation with me. This is the exact opposite of how Christmas went and I was hurt and confused. After they left town the next day I asked him if he had any idea what was going on and he told me how he only really talked to them about me when things weren't perfect and they now hated me. I had never hurt or abused him, so I was understandably flabbergasted at how their opinion of me had changed so much especially when they never even interacted with me aside from when I was staying with them for Christmas.

I broke up with him a few months later for mostly unrelated reasons. I'd be lying, however, if I tried to say that his family now hating me didn't play into it at all. When I broke up with him he mentioned that he was already looking for a ring for me. (Maybe he thought I was breaking up with him because he hadn't proposed yet? I don't know...) That knowledge sat with me for a while and I just couldn't stop thinking about how I would have felt if we hadn't broken up and I married someone whose family hated me. It would have been horrible for me.

You're not me, so your decision and thought process won't be the same as mine. All I can do is pass on my experience and what I learned about myself in that process. If you really do feel like he is your person and you both absolutely want to move forward with a life together, then there are some discussions that you'll really need to have. You'll have to come to an agreement on what things can be shared freely and what needs to stay between you guys or be shared only with a professional or other completely unbiased party. If you want a good relationship with his mom and sister, then you'll need to figure out the best way to move forward to try to make that happen. However, they are their own people and don't have to have a relationship with you if they don't want to. If they are unwilling, is that something you could be okay with? Are you able to acknowledge the awkwardness but not let it take over or try to talk about it every time you see them? There are really a lot of factors at play here. My best advice would be to take the time you need to really figure out if this can work for you guys. And I would suggest figuring that out before moving forward with any wedding planning.

I absolutely wish and hope the best for you. These situations are never fun and I'm so sorry that's where you are now. Best of luck, and my thoughts and prayers are with you!

"But he decided to propose anyways and wants to talk to his family about it next week."

GIRL! RUN GIRL RUN!!!! He has to talk it over with his family? The family that does not like you because of HIS oversharing? BREAK UP NOW AND DO NOT LOOK BACK. I am 56, I have seen a lot of stuff (including a lot of stuff like this) and it NEVER ends well. He won't respect boundaries, he does NOT respect you or your relationship. Trust me and good luck!