How do I get my boyfriend to stop telling his family about our relationship problems?

Anonymous
We have been together for 18 months in the beginning I use to tell my family about our relationship problems. but stopped because i realized my relationship problems was none of there business and once they meet him they was not going to like him if i continued telling them about our relationship problems. lately my boyfriend has been telling his brother and sister about our relationship problems i even went through my boyfriends phone one day and he sent his brother a Screenshot of a message I sent to him so now I'm uncomfortable sending him messages if we ever get into a argument. He asked me a few times to meet his family I told him no because he keeps telling his brother and sister our business and they might not like me after all the things my boyfriend has told them how can I get him to stop telling his family our business i already asked him to stop but he just keeps running his mouth
How do I get my boyfriend to stop telling his family about our relationship problems?
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