OK, I am going to go against he grain here and try a different point of view.
If you marry this man, who you love, you will have SIL for ever.
If this situation goes on, you will have tension in your marriage for ever.
For the sake of the love you have for your husband to be, can you PRETEND that you love your SIL. I know you don't, but can you 'fake it till you make it'?
Can you rise above the petty snipes and rubbish on Facebook and try to see her as someone who loves your boyfriend, who would like to be an indulgent auntie to your DC, and someone who is going to be part of your life for a long time.
She might not know how to behave with dignity and kindness, but you can.
Even if sometimes it is through gritted teeth.
yes it might seem 'false' to you, and not 'honest', but there is some big stuff at stake, and it might save your relationship, and actually make you happier, if you can try to be the bigger person. It will also give no ammunition to your SIL if you are genuinely trying to see her point of view and to be kind to her.
I understand all the posters saying 'stand up for yourself', and in most things I would agree, but this is family, and the feelings clearly run very deep.
My granny used to say 'YOu catch more flies with honey than with vinegar'
Good luck Tilly whichever way you play it