AITAH for telling my SIL’s Fiancé he is being a child?

Myself (24M) and my wife (25F) moved halfway across the country a while back for my work. Her sister (28F) had their wedding planned for the summer after we moved, to her fiance (31M) of 12 years. A week before the wedding they canceled it due to a long history of cheating, differing views, and countless other issues. Mind you they have 3 children together. After they broke it off she remained silent a cordial but her fiancé decided to take to social media and talk bad on her and the family. My wife told me when we started dating that I needed to be involved with the family, which is not something I wasn’t used to, coming from a military family. Nonetheless, I did my best and have a great relationship with her family. When i saw him talking crap online calling her the C word, B word, terrible mother, and all the hurtful things he could, I called him up and basically told him to cut it out. His response to me was “stay out of my family business”. I felt as if he made it my business when he posted about OUR family.

Now that I’m in town he is not only avoiding my wife and I but our family too. That is because “he is going to fight me” according to her sister. Not sure how to feel about it but i think this reinforces the fact that I called him childish.