How do you deal with SO's crazy family?

So yesterday my SO's sister snapped on him, in the middle of a restaurant, about how he had no right to discipline his daughter over something she (sister) told her (daughter) she could do. Me, being quick on the draw, immediately shot back that the daughter needs to check in with US and ask for permission, not her. Because we had zero clue that the sister had told her it was ok to defy us like she did. After I said those few simple words, all hell broke loose and SO and I were degraded by his sisters boyfriend. I told him to parent his kids how he sees fit and to leave us to ours. My SO reiterated this as well. So now all last night, and publicly on FB today they have been harassing us. SO handled it when they spoke to him... and when his sister went off on me on FB saying I was a pretend step mother and had no business saying a word to anybody about anything... I asked SO to deal with it. Well he did, after we fought over it... he says I should handle it myself because I put myself out there to start with and it's not fair for him to be in the middle of me and family. I'm so torn... I thought I did the right thing and stood up for my little family, but it's resented. 😢 And now she's demanding an apology from me and I could just scream. I've said all of 4 sentences to anybody, no name calling, not even being a smart ass, and I'm out of line?? While they attacked me personally, my parenting, and my position in my own family?? SO says I should too. Note... She's always treated her brother (my SO) like crap, last time she attacked his character and went psycho on him I warned him if it happened in front of me I wouldn't stay silent, and I think I handled it very well... And then I am told I should have just kept quiet. 😞