NTA

I think in her heart she understands but a wedding only happens once. She needs to calm down but also I think she’s more mad at the situation, not you.

I also think if your son felt uneasy before the wedding alternative plans should have been made. Your son is important snd you did the right thing but the night of your stepdaughters wedding probably wasn’t the best time to take a risk (again, not saying you should have stayed, just saying that maybe there should have been 2-3 backup plans since both are so important in their own way). I don’t know what your son went through (that will make it make more sense from our view). Does stepdaughter know the details of why your son is having a hard time?

Possible idea: because I’m a mush for sentimentality, make the living room fancy with lights and curtains and have your own little father daughter dance. Surprise her with dinner or maybe a mini wedding cake, get her in her wedding dress, all of that stuff. A little cheesy, yes. Memorable and a genuine gesture, yes. Something that lets her know that you care and you hated having to choose, but if someone you love is in trouble, that’s the only thing that Matters at that moment.