I hope you never let her near your children. She's toxic. Every sentence in her letter is dripping with emotional blackmail and trying to put you on a guilt trip.
I might be inclined to reply to her, reminding her that it was a non-JW party and that you were an invited guest too. Neither she, nor any other JW attending, had more right to be there than you did. If she is unable to get along with everyone at the party, then SHE should have stayed home. Her tears were probably because she was pissed that you didn't disclose the pregnancy to her, the way a daughter would do with a NORMAL mother. She is the one with the problem, not you, she's the one with a flawed sense of morality that gives her permission to judge people for things that are none of her business. Why would you confide this happy event with her, when you knew she would disapprove, judge and reject you the way she has? I wonder what other people at the party were thinking, to see a mother IGNORING her pregnant daughter??
Enjoy your pregnancy. Enjoy your baby. Your mother doesn't deserve to be part of your life or your baby's, not with that attitude.