Thanks ladies for your support and advice in my thread below. I had NO idea that my family would be this awful!
After sleeping on what you all said, I have decided:
1. I do need to rescind these invitations personally. I would like advice on an appropriate way to do that, which effectively communicates that their cruelty isn't tolerated. The wedding is October 16th, but I think it still needs to happen. Also, I will be making it clear to my parents/siblings that these relatives are to know NOTHING about my life, husband, or children from here on out.
2. I need a way to discuss this situaiton with FI's mother, who is the most amazing FMIL I could've asked for. She has been very much looking forward to meeting my family... and now will not be meeting 90% of them. Since she's very sensitive, I feel I need to say something to her to explain the poor turnout. When FI and I started dating, she expressed apprehension to me about how my family would treat/whether they would accept him; unlike me FI's neither white nor American-born, he and his family experienced a fair amount of xenophobia/racism when they first moved to the US. I need her to know that it's 100% about my marital status, and 0% about the amount of melanin in her son's skin. I need suggestions from clearer heads on broaching this topic without sounding all "poor me".
TIA.