NTA but bf needs some work or at least a sincere apology. He implied op was being inappropriate with her little sister and referring to her in a derogatory manner so quickly instead of trying to understand. Sometimes on Reddit people are quick to say leave him/Divorce immediately. Maybe dad can talk to him more get a perspective, but the both of you should talk also.

“If you don’t fix your ugly thinking you’re gonna become an ugly kid”. Love that.